Why does my RF meter show the same readings even if I surround it with aluminium foil?

I've been thinking about putting up some foil to protect my bedroom from microwaves.

But strangely, foil seems to have no effect on my RF meter. The reading always stays the same, even when I  try surrounding it with various types of foil, or a metal enclosure like a pan. I only leave a very small gap, just so I can see the meter.

Why is it not changing?

Not having all the details here of your experiment I can only guess.  Your RF meter has its own antenna inside to pick up external RF waves.  When holding the meter close to metal objects you may actually be picking up the signal emitted by its own antenna.

Interesting theory. I find that unlikely because the reading just doesn't seem to change at all.

I'm wondering if maybe it's just a bad meter? I've just made a new thread about my meter here:
https://www.es-forum.com/Opinions-on-RF-meters-Not-happy-with-my-GQ-EMF-380-td4036804.html

I have heard that if its not grounded aluminum can act like a antennae. Not an expert. Also I think you have to be careful about which side of the aluminum is facing out (shiny smooth side) against what your blocking otherwise again it acts more like a antennae.