Light flashing from top left corner of 11pro max that can only be seen on our security cameras

Hi. My family all have iPhone 11 pro Max’s. Every few seconds 2 of our phones are flashing but it is only visible with our security camera. The other 2 phones don’t do it at all.

any suggestions as to what this could be?

I can’t add a pic as it’s not visible. Also - the phone will do it is sitting on a table or in my hands - so it’s not the proximity sensor..