Clothing with sewn in RFID security tags, rather than the pin on plastic type removed at the store, or ones stuck to the hang tags. Very commonly used by the Gap, Old Navy, and Banana Republic. Despite the "remove before wearing" printed on them, they're very frequently not noticed in items made of heavier material, particularly pants, until you go through a security gate at a store and beep. First you wave any bags you're carrying through, and find nothing beeps.
Then you take your cellphone out of your pocket. It's not that.
Then your keys.
Then your change.
Frustrated, embarassed, and completely bewildered at this time as to why you're still setting it off, you'll often still get through with just a hairy eyeball
after they've seen enough of your stuff that they can tell you're probably not shoplifting, but it'll happen again, and again, until one day you accidentally stumble across the tag.
Trackerpants. They're out to get you.
(a common contributor to securapathy
among shop staff)