While you are stealthily moving around the map always keep your eye on the meter bar when it appears above your head because if it becomes full and turns red you will only have a few seconds to perform the following two things to stay alive before the T-Rex finds and tramples you.
1) Exit and re-enter the map.
2) Close the Jurassic Survival app entirely and reopen it (this could be considered cheating).
Note: If one of the two T-Rex has locked onto you you will be able to run past the other T-Rex on the map undetected.