Well, after playing some more, i have found out that the survivors only use their turbo engine (mostly westbound for some reason...) when the time compression is 8x or higher, both in 3D view (bridge for example) or in/on nav map screen.
When i remain at TC 4x or less, the survivors stay in the same place where i first spot them.
Maybe some modder with insight in the program routine regarding time compression knows why 4xTC/8xTC is the magic 'switch' ? Coriolis/earth rotation maybe?
Do the survivors try to stay in the same time zone when TC is 8x or higher?
Happy hunting,
@lantis