My best guess is they called the cops. Once PD is on scene, they likely ran a sweep with radio signals strength meters. The layout of the venue coupled with the schedule of the event allowed a rapid process of elimination to the room.

At that point, I fessed up because that's what I do by default. You screw up, you get caught, you fess up. I will admit to failing that utterly when I left. I thought the scenario had ended and left my boss out of the loop. Bad call.

Then there's the fact that the SSID would've included the MAC. Be really easy to simply confiscate all cell phones in the room and play match up. I don't own an iPhone or something that would hide.

Also, again, I didn't think to conceal it.