[walrus01]

For example an imsi catcher spliced into 120vac power and stuffed up into a ceiling tile in a busy shopping mall. I bet that with a stepladder, a clipboard and two guys in high visibility vests, a set of electrician tools, you could do this at nearly any mall in America.

[subway]

Bonus points for installing a few near each other, configured to operate identically, but at different times.

[mschuster91]

How do you get the sniffed data, though? You're gonna need some form of high bandwidth uplink.

[OnMyPhone]

One option might be to have the sniffing device setup with a hidden wireless network (of some sort).

Then when you walk around, sit in the food court eating, or if the device is close to the outside, sit somewhere out of sight of the cameras with a strong wifi antennae and grab whatever data.

Unless they have some "free" wifi that the device could hop on to send it to some server somewhere.

Using that and / or 4g would make it easier to find the device and, of course, the person though.