Tile, or Trackr, both have this ability. I accidentally dropped my house keys that had a Tile attached. Somebody found the keys, and handed them into a Tesco branch that was close to where I dropped them. The Tile connected to their instore wifi, and I was able to locate the keys. Mind you, the location map went crazy, because I think the Tile "locked-on" to the Bluetooth of some customer's phones, so it was also indicating that my keys were lying somewhere in the store car park! I think after a while (could be because the signal it had locked on to had moved out of range), the device reconnected to the store WiFi. I went back to the store, and enquired at the customer service desk if a set of keys had been handed in. The girl said she had a few sets of lost keys, so I activated the ringer on the phone app, and my keys were identified. The staff were certainly impressed with this device! They asked about it, and how it works, so I wouldn't be surprised if they bought a couple for themselves.
As an experiment, why don't you leave a smart tag in the desk drawer at work, or with a relative, and see if it connects with the nearest WiFi, then try to locate it at home. I think it should show its location fairly accurately, otherwise its a not-so-smart tag. I think you are able to ring the tag once you are close by, so that you can locate it audibly. I think you can also use the Smart Tag to ring your misplaced phone - Tile and Trackr both have this function.
