iFixit has a nice teardown of what, one model, of an FBI GPS tracker look like. It’s really fascinating and if this is what the FBI typically uses — well it would be a cinch to spot under your car to even an untrained eye. Having said all that I am supremely disappointed with the use of D-Cell batteries, they are basically sticking a [Mag-Lite](http://www.maglite.com/product.asp?psc=6DCELL) under your car — that’s going to effect your fuel mileage.
