I'm struggling to put into words what I'm seeing, but here's my best attempt.
I'm writing a simply BLE scanner that uses callbacks, based on the example code. I've made it as simple as possible, but it's not acting the way I'd expect.

The code:

#include "NimBLEDevice.h"

NimBLEScan* pBLEScan;
long lastUpdate;
const int scanTime = 3;

class MyAdvertisedDeviceCallbacks: public NimBLEAdvertisedDeviceCallbacks {
    void onResult(NimBLEAdvertisedDevice* advertisedDevice) {
      Serial.printf("Advertised Device: %s \n", advertisedDevice->toString().c_str());
    }
};

void setup() {
  Serial.begin(115200);
  BLEDevice::init("");
  pBLEScan = BLEDevice::getScan(); //create new scan
  pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks());
  pBLEScan->setActiveScan(true); //active scan uses more power, but get results faster
  pBLEScan->setInterval(97);
  pBLEScan->setWindow(37); // less or equal setInterval value
  pBLEScan->setMaxResults(0);
}

void loop() {
  long now = millis();
  if ( now >= lastUpdate + 1000 ) {
    Serial.printf("Tick: %ld\n", now / 1000 );
    lastUpdate = now;
  }
  BLEScanResults foundDevices = pBLEScan->start(scanTime, false);
  pBLEScan->clearResults(); // delete results fromBLEScan buffer to release memory
}

What I want to see is "Advertised Device: XXX" information printed periodically, as devices are discovered. I'd also like to see the word "Tick" posted every second, indicating that the scan is non-blocking and happening "in the background" while my main loop() continues to work.
In this case, it seems that the pBLEScan->start() is blocking, and since my scanTime is 3 seconds, I see "Tick 3" ... "Tick 6" ... "Tick 9"... and then... nothing. The output looks like this:

15:26:53.409 -> ets Jun 8 2016 00:22:57
15:26:53.409 ->
15:26:53.409 -> rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
15:26:53.409 -> configsip: 0, SPIWP:0xee
15:26:53.409 -> clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
15:26:53.409 -> mode:DIO, clock div:1
15:26:53.409 -> load:0x3fff0030,len:1420
15:26:53.409 -> ho 0 tail 12 room 4
15:26:53.409 -> load:0x40078000,len:13540
15:26:53.409 -> load:0x40080400,len:3604
15:26:53.409 -> entry 0x400805f0
15:26:54.044 -> Advertised Device: Name: 846B215F5FC233AEE9, Address: f3:33:c0:55:58:95, appearance: 0, manufacturer data: a705051001000000000000001b00ca, serviceUUID: 0xfe07, txPower: 0
15:26:54.508 -> Advertised Device: Name: , Address: 75:6d:2b:0e:f5:29, manufacturer data: 4c001006191e23cfe2c1, txPower: 8
15:26:55.115 -> Advertised Device: Name: , Address: 76:90:c3:e9:90:02, manufacturer data: e00001b9ca72c54c, serviceUUID: 0xfe9f
15:26:55.115 -> Service Data:
15:26:55.115 -> UUID: 0xfe9f, Data: �nTF065F6mZs
15:26:55.115 -> Advertised Device: Name: , Address: 43:16:54:3d:42:ed, manufacturer data: 4c001005491c18dbfe
15:26:55.221 -> Advertised Device: Name: , Address: d7:96:ba:2b:33:e2, manufacturer data: 4c0012020000
15:26:56.187 -> Advertised Device: Name: , Address: 5d:0c:66:6f:61:2e, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:26:56.664 -> Advertised Device: Name: , Address: 5a:35:34:88:dc:8a, serviceUUID: 0xfd6f
15:26:56.664 -> Service Data:
15:26:56.664 -> UUID: 0xfd6f, Data: ⸮<⸮�
15:26:56.912 -> Advertised Device: Name: , Address: 4b:21:d4:48:87:b6, serviceUUID: 0xfe9f
15:26:56.912 -> Service Data:
15:26:56.912 -> UUID: 0xfe9f, Data:
> 15:26:56.912 -> Tick: 3
15:26:56.950 -> Advertised Device: Name: , Address: 43:16:54:3d:42:ed, manufacturer data: 4c001005491c18dbfe
15:26:56.950 -> Advertised Device: Name: , Address: 5a:35:34:88:dc:8a, serviceUUID: 0xfd6f
15:26:56.950 -> Service Data:
15:26:56.950 -> UUID: 0xfd6f, Data: ⸮<⸮�
15:26:58.214 -> Advertised Device: Name: , Address: 76:90:c3:e9:90:02, manufacturer data: e00001b9ca72c54c, serviceUUID: 0xfe9f
15:26:58.214 -> Service Data:
15:26:58.214 -> UUID: 0xfe9f, Data: �nTF065F6mZs
15:26:59.290 -> Advertised Device: Name: , Address: 5d:0c:66:6f:61:2e, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:26:59.928 -> Advertised Device: Name: , Address: 75:6d:2b:0e:f5:29, manufacturer data: 4c001006191e23cfe2c1, txPower: 8
15:26:59.928 -> Advertised Device: Name: , Address: f3:33:c0:55:58:95, appearance: 0, manufacturer data: a705051001000000000000001b00ca, serviceUUID: 0xfe07, txPower: 0
> 15:26:59.964 -> Tick: 6
15:27:00.073 -> Advertised Device: Name: , Address: 43:16:54:3d:42:ed, manufacturer data: 4c001005491c18dbfe
15:27:00.682 -> Advertised Device: Name: , Address: 5a:35:34:88:dc:8a, serviceUUID: 0xfd6f
15:27:00.682 -> Service Data:
15:27:00.682 -> UUID: 0xfd6f, Data: ⸮<⸮�
15:27:01.043 -> Advertised Device: Name: , Address: 76:90:c3:e9:90:02, manufacturer data: e00001b9ca72c54c, serviceUUID: 0xfe9f
15:27:01.077 -> Service Data:
15:27:01.077 -> UUID: 0xfe9f, Data: �nTF065F6mZs
15:27:01.328 -> Advertised Device: Name: , Address: 5d:0c:66:6f:61:2e, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:27:02.952 -> Advertised Device: Name: , Address: f3:33:c0:55:58:95, appearance: 0, manufacturer data: a705051001000000000000001b00ca, serviceUUID: 0xfe07, txPower: 0
15:27:02.952 -> Advertised Device: Name: , Address: 75:6d:2b:0e:f5:29, manufacturer data: 4c001006191e23cfe2c1, txPower: 8
> 15:27:02.988 -> Tick: 9
15:27:02.988 -> Advertised Device: Name: , Address: 43:16:54:3d:42:ed, manufacturer data: 4c001005491c18dbfe
15:27:03.388 -> Advertised Device: Name: , Address: 5d:0c:66:6f:61:2e, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:27:04.965 -> Advertised Device: Name: , Address: 5a:35:34:88:dc:8a, serviceUUID: 0xfd6f
15:27:04.965 -> Service Data:
15:27:04.965 -> UUID: 0xfd6f, Data: ⸮<⸮�
15:27:05.430 -> Advertised Device: Name: , Address: 76:90:c3:e9:90:02, manufacturer data: e00001b9ca72c54c, serviceUUID: 0xfe9f
15:27:05.430 -> Service Data:
15:27:05.430 -> UUID: 0xfe9f, Data: �nTF065F6mZs
15:27:05.430 -> Advertised Device: Name: 846B215F5FC233AEE9, Address: f3:33:c0:55:58:95, appearance: 0, manufacturer data: a705051001000000000000001b00ca, serviceUUID: 0xfe07, txPower: 0
15:27:09.114 -> Advertised Device: Name: , Address: 75:6d:2b:0e:f5:29, manufacturer data: 4c001006191e23cfe2c1, txPower: 8
15:27:17.923 -> Advertised Device: Name: , Address: 7e:d5:fc:b0:9c:b5, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:27:35.274 -> Advertised Device: Name: , Address: d7:96:ba:2b:33:e2, manufacturer data: 4c0012020000
15:27:38.533 -> Advertised Device: Name: PROV_005d13Z, Address: 24:62:ab:e2:16:ee, serviceUUID: 0000ffff-0000-1000-8000-00805f9b34fb
15:27:49.808 -> Advertised Device: Name: , Address: 86:2a:fd:a8:50:c6, manufacturer data: 650001c905, serviceUUID: 0xfe78
15:27:49.808 -> Service Data:
15:27:49.808 -> UUID: 0xfdf7, Data: �⸮X⸮⸮⸮⸮V⸮⸮ziEfs�5
15:27:54.071 -> Advertised Device: Name: , Address: 43:16:54:3d:42:ed, manufacturer data: 4c001005491c18dbfe
15:27:54.360 -> Advertised Device: Name: , Address: 76:90:c3:e9:90:02, manufacturer data: e00001b9ca72c54c, serviceUUID: 0xfe9f
15:27:54.360 -> Service Data:
15:27:54.360 -> UUID: 0xfe9f, Data: �nTF065F6mZs
15:27:55.217 -> Advertised Device: Name: , Address: 7e:d5:fc:b0:9c:b5, manufacturer data: 4c00021561b5c6c7bc4249c982cc3fc6832f69ae00640001a6
15:27:57.163 -> Advertised Device: Name: PROV_005d13Z, Address: 24:62:ab:e2:16:ee, serviceUUID: 0000ffff-0000-1000-8000-00805f9b34fb
15:28:00.645 -> Advertised Device: Name: 846B215F5FC233AEE9, Address: f3:33:c0:55:58:95, appearance: 0, manufacturer data: a705051001000000000000001b00ca, serviceUUID: 0xfe07, txPower: 0
15:28:06.468 -> Advertised Device: Name: , Address: 75:6d:2b:0e:f5:29, manufacturer data: 4c001006191e23cfe2c1, txPower: 8
15:28:09.374 -> Advertised Device: Name: , Address: d7:96:ba:2b:33:e2, manufacturer data: 4c0012020000
15:28:09.661 -> Advertised Device: Name: , Address: 4b:21:d4:48:87:b6, manufacturer data: e00001f2ca849dc4, serviceUUID: 0xfe9f
15:28:09.661 -> Service Data:
15:28:09.661 -> UUID: 0xfe9f, Data:

Reading through that, you see I get a "tick" at 15:26:56.912, at 15:26:59.964, 15:27:02.988, and ... never again for the next 60+ seconds. The scan appears to continue to run, but my main loop() just... disappears.
Questions:

• where is executing disappearing to, that my simple little loop() stops looping?
• am I unrealistic in thinking there's a way to scan and continue to process at the same time. Even with the callback, the scan appears to block execution of the main loop

BTW, I don't think this is a "bug" or "issue", but I couldn't find a more appropriate forum to ask this question. If there is one, let me know and I'll move there. Thanks!

John