Originally Posted by contango

It's unlikely that any device would be particularly effective.

What you're describing is something that can hide on the bike unseen by a thief, and track its own location with GPS which is relatively light on battery. But then it has to broadcast its location so you'll effectively need it to be a very compact cellphone (complete with SIM card etc) in order to be able to actually know where it is at any given point.

If it's taken indoors (or put in the back of a box van or similar) you could lose GPS coverage, in which case it won't know where it is. If someone jumps on the bike and rides off with it you might be able to track them for a while but if they do something as simple as putting it in the back of a van or wheeling it indoors you'll almost certainly lose its location.

Unless you're talking about a particularly expensive bike my thoughts are that it would be easier to just buy a decent lock to discourage people from stealing it. And if the bike is particularly expensive it's probably a better option to save it for rides when you won't be leaving it, and buy a cheap bike for when you will.