I suspect you're thinking of one of the newer generation of Bluetooth Low Energy (BLE) trackers, Like Tile or Chipolo. They're cheap, small, and have low battery consumption. The problem with them is they're BLE, not GPS, which means they have a very limited range.
Your phone - the BLE receiver - needs to be in range to locate the tag. Which is not much use if some scrote has whisked your Pretty's away to his/her fetid cave. BLE tracking services rely on a crowd concept - a whole bunch of users wandering around with their app running on their phone. If your tagged tool is stolen, then it needs someone in that crowd to pass close enough to the stolen device for their phone to detect the tag, and then alert you.
I'm not sure how much penetration these crowd networks have at the moment- I looked at the tech when it was just getting off the ground, and decided it wasn't worth it at the time. This may have changed as popularity increases, and is much more likely to have achieved a useful coverage in the UK where your population density is quite high. There was some muttering about UK police carrying detectors, but I'm not sure what (if anything) happened to that.
The alternative is a true GPS device, but these are more expensive, bulky, and draw much more power, so battery life and recharging become an issue. They're great for cars as they can be hidden, and have a big power source to draw from. Less useful in a systainer, but some cunning engineering could see you fit one with a disguised charging connector.
At some point, the two techs will combine in some way and we'll get that cheap, low power, fully trackable device, but it's not here yet.
