when was the last time the tracker battery was replaced - if thats more then about 4 years old it could have gone duff and be pulling a fair old load trying to keep itself charged.
the other way is to disconnect your neg lead and place your ammeter in series as your were doing. and then pull the fuses out the box , one at a time, and see if the current drops.
on my escos, it turned out to be the facrory alarm.
i didnt leave the car locked in my old secure garage. turns out the whole draw on the car battery then left unlocked was around 350ma. when locked this droped to 50ma for the whole car.