The FCC could just park a van in front of the store for 3 minutes and their tools would verify that a jammer was in use.

Here's the details on anti-jamming laws, and a list of people who have had their ass handed to them by the FCC for doing it: https://www.fcc.gov/general/jammer-enforcement

Would be interesting to try to detect if such interference is present on the right frequencies, or if it is just a natural effect and truly not intentional.

Thanks for sharing the info.

That pretty much describes 99% of indoor mobile coverage out there. The only exception being buildings that have ponied up for a femtocell.

Actual jammers? Unlikely because it's highly illegal, quite detectable, and will cost more than anyone could hope to gain by it.

Deliberate, passive measures that practically impede indoor reception may happen, though.

It does seem that some buildings are suspiciously effective Faraday cages while others are not.

Could it be something that makes the cell network jittery or adds latency that interferes with cellular data but not voice service?