Strange lights in night IR mode?

I've just setup a 7616 NVR with 2032 bullets. On one camera I'm getting strange dancing lights - looks like some kind of reflection. See attached picture. These lights flicker, and dance around slightly - triggering motion detection. The area is totally dark - we have no street lights in our area and nor are there any visible lights from neighbouring properties.

Any ideas what causes it and how to avoid it?

Also any hints on minimising false triggers from bats (maybe not) and...

Strange lights in night IR mode?