Some remotes support polling for changes on them. As far as I can tell, only drive does this now.
cache can subscribe to those events and automatically expire its entries too.

Some issues I found already:

• moving a file doesn't seem to expire both parent folders, only the destination. At the moment this makes things a bit tricky as cache would best work if it doesn't expire its entries at all but it seems some cases aren't included in the notifications
• I noticed that the notification is sent to entries which are included in the global dircache that the drive remote uses to translate between a drive ID and a path in rclone. I haven't seen an issue yet but I'm wondering if there might a case where DirCache will not have an entry while it does exist in cache.