Yep. As @Crafter_Man says in post #13, his overgrown metal shed does the same thing.
Never attribute to malice what can be explained by economy = cheapness.
@chela your car/system doesn’t auto decrease volume at braking, does it?
If I don’t brake in time the alarms will go off and that’s all I’ll hear. 
I’ve goofed with jamming devices and they’re good for that but there’s no way a theater would install one. I could more easily imagine an employee or even a patron carrying/installing? something covert but I really doubt there’s anything actively nefarious at all.
Right. If the architects, engineers and construction people are halfway smart about things, it would be easy enough to spec designs and materials that effectively reduce cell signals inside a building while still maintaining plausible deniability.
From a little googling, it looks like the challenge is more in not reducing cell signals than the opposite.
I don’t think they even need to maintain plausible deniability. “Our guests don’t want to be annoyed by phones ringing during the movie, so we chose building materials that would block cell phone signals”.
Off-topic, hidden
It might be best to avoid using cell phones in theaters anyway.
It might you get shot even if you aren’t the one using the cell phone!
From Killing of Chad Oulson - Wikipedia
Unfortunately, the shooter was found not guilty. He should be spending his golden years in prison. And Oulson was hardly innocent. He should have kept the cell phone off.
[Moderating]
@billy-jack , this is not the thread for discussion of that incident. If you wish to discuss it, find an existing thread about it, or if there isn’t one, start a new one yourself, in the appropriate forum. What forum is appropriate depends on the sort of discussion you want: If you just want to report the facts of the case, MPSIMS would be appropriate, while if you wish to complain about the stupidity or malice of those involved, the BBQ Pit would be appropriate.