The Mu Tags are tiny “loss prevention” smart-tags that track the location of the items they are placed on. Mu Tags even allow you to create safe-zones, so when a certain tag leaves the zone, you’re alerted immediately. Each tag is ridiculously small (the size of your thumbnail), making it ideal to snap onto everyday items, and comes with a battery that can run for 3 months on a full charge.
Let’s not get into the legality of this device. But if you have a need to listen in on conversations for whatever reason, the Spy Bug Audio Surveillance gadget pictured above should do the trick. About as small as a pack of matches, it accepts a SIM card, which means you can just call it anytime you want to eavesdrop. Better yet, you can set it so that anytime there’s a sound above 45dB within 10m, it calls you! $60