they would have to be active rfid tags. i've been using some active tags from http://goo.gl/DkZkB for vehicle home/away detection for the past 5 years. they aren't quarter sized, but they're in the glove box out of sight. they work well.
i'm also using a network of hacked seagate dockstars w/ bluetooth usb dongles hidden around the house to detect which part of the house our cellphones are in.
i'm already plotting what i can use these stickers for :)
