Trax does not work indoors.
If you enter a building or a tunnel, the tracker will lose its GPS signal. Once you go outdoors again, it will resume tracking.
When the Trax is on and indoors, it cannot recognise that it is in a building. This means that it will keep trying to update its GPS position, but as it cannot connect properly to the satellites, it might indicate an inaccurate location.
This may give the impression that the tracker is "jumping around" on the map. This does not mean that the tracker has a problem, but simply means that the GPS signal is too low at that moment to give an accurate location.
Just remember that Trax works outside, but it cannot be used indoors!