Ballys and Paris began using these devices in a promotion last week. I believe something similar was used a year or so ago but I never was around to pick one up. A little smaller than a slot card, but about 3/8" thick, these have some kind of electronics built in that picks up a signal sent throughout the casino. I didn't stick around the whole period (ends at 5 p.m. every day) so mine didn't light up. It appears the light comes from the three holes on the front side.

I have an extra one to trade for a non-Nevada slot card that I don't have.