the device your looking for is called a clip on ammeter,but if youve got an okayish multimeter,you`ll have an ammeter on that. I`m with tabetha that its highly unlikely to be a tracker causing it,mine wasnt activated on the scooby for years and it never drained my battery once,again with a cat 1 etc on it.

The ammeter section of your meter usually requires you to swap the red lead into a different part of your multimeter and you then put it into series with your car battery to see how much current your battery is draining.

If you then start to remove fuses the current should drop off when you find the faulty circuit and then you can trace it back from there.Usually its something like a boot light staying on but i`ve known heated rear screens to get stuck on with a faulty switch contact but not display on the switch/dash etc.An old sound system not removed correctly can also cause probs.