proximity sensor

There is no light that indicates the proximity sensor has been triggered. If it's triggered, the screen goes black.

There is an orange (or red depending on the display settings) light that tells you if your microphone is in use. Is that what you're seeing?

There is no flashing light that indicates the proximity sensor is being used.

I'd suggest that you contact Apple for hardware support:

Genius Bar Reservation and Apple Support Options - Apple

That is the IR used for Face ID. Eyeballs can't see it.

You can see it using a camera.

QuickTimeKirk wrote:

That is the IR used for Face ID. Eyeballs can't see it.

Perhaps the OP is a cat? I think they can see in the IR range.

Ah!

Now I'm forced to wonder why one would be looking through a camera at a phone but, I'm sure there are reasons.