Central Locking / alarm sensor issue

So - I've been struggling with this for a while:

Stand and press the button on the keyfob while standing a few inches from the car near either driver or passenger door.
I get a 1 in 5 best success rate. Sometimes it just won't play ball - lock or unlock.

Other keyfob is the same. Have changes batteries.

Today - I made a discovery (nice pun..) and if I hold the fob in the middle of windscreen (outside) pointing down at the windscreen air vent it works 100% of the time!!

Looking at the very centre of the airvent just in front of the glass (inside) there is a little hole that looks as though it should have sensor in it. I can imaging a little dome shaped thing that should be there?

Has my sensor fallen down inside the vent? (assuming this is where it lives??)

I fiddled about under the dash both sides and behind the stereo but there is no access in there so before I go further, am I looking for something that should be there? Or in completely the wrong area - I had a surf for pics of the dash there but it's inconclusive.

If so, any ideas of retrieving it without taking the whole dash out??

ideas welcome...

Hi, usually the little dome shaped sensor on the windscreen is a sun sensor usually fitted to the car if it has climate control to help the car monitor the inside temperature , so if the car has climate control then this is what its for if its no climate then it would just normally have a blanking plug fitted

yeah - starting to think that as I check more pics.

So - maybe easier this way:

When you click the keyfob button to open the doors, where is the sensor for it in the car?