Peeled the price sticker off my muffin box and found a whole circuit under there

I ran into the same thing last year when I bought a pack of socks and noticed a little gray loop under the label. I work part time at a hardware store so I'd seen the handheld readers we use for stock counts, but I never put two and two togather untill I tore a tag apart on my lunch break. Under the printed layer there's just a chip and a flat coil, nothing else. No battery at all, like you said.

The part that surprised me was the range. Our store reader only picks tags up from a couple feet away, so I figured they were short range devices, but the same kind of tag can be read alot farther with a bigger antenna. Depends entirely on the reader, not the tag. The grease point is real too. We had a pallet of motor oil that leaked and the labels got soaked, and those tags still scanned fine. The coil is printed on plastic film under the paper so it doesn't care about a little moisture. I've stopped peeling them off my stuff out of curiosity and started just leaving them alone unless they're somewhere visible.

I actualy tried reading one of those tags with my phone after noticing the same coil pattern on a package of razors. My phone has NFC and I figured since they share a frequency it might pick something up, but all I got was a generic ID number with no useful data in it. The store tags are locked down and only the retailer's own readers can pull anything meaningful out of them. Where I did find them interesting was at home. I bought a pack of blank stickers online for a few bucks and stuck them on my toolbox, the recycling bins, and my ski boots. Tap the phone and it flips a smart light on or opens a note. Same basic loop antenna, same 13.56 MHz band, just programmed for my use instead of inventory.

One thing I noticed that backs up your grease observation: these things are tough. I had one on a garden tool handle that sat outside all summer through rain and sun, and it still read fine. The antenna traces are etched in aluminum or silver ink under the printed layer, so there's not much to corrode. I did learn the hard way that they hate metal surfaces though. Slapped one on the fridge and it went completly dead until I put a thicker spacer behind it.

Same discovery here but from the opposite direction, my kid stuck one of those shiny stickers on the fridge and it wouldn't come off the door sensor list at the library where he'd swapped it onto his book. Anyway, one thing I've wondered, do the checkout scanners actually deactivate these or just unlock the door gate thing? I've walked out with a tagged item that still beeped the pad a week later at a diferent store entrance, which makes me think they're alive until the chip gets a kill command. Might try the microwave trick on one, though I've heard the coil just arcs and the chip survives.

Reminds me of the early 90s when I was stocking nights and everything got a price gun sticker. We thought THAT was high tech. One thing I figured out on my own: snip the loop anywhere with scissors and the tag is dead, no reader will ever see it again. Did that to a jacket tag once just to prove it to a skeptical coworker. Also if you're worried about someone walking past your house with a reader, these things need to be pretty close to pick anything up, unlike the old license plate toll transponders my buddy swears broadcast a mile.

I agree with the point about the store tags being locked down so you can't pull useful data with your phone. It seems like they are only meant for the system they were put into. I saw one of those coil tags on a pair of returned power tools once, which was weird. I thought they were strictly food or clothing items. The chip itself was mostly obscured by a small piece of adhesive residue, but I could still clearly see the entire thin metal loop underneath the sticker. It was definately not just part of the packaging, it was the product itself.