I have a similar question after reading Tunnel 29, just beyond the immediate guard (VoPo) encounter. Considering these 3 facta:
Amost all the successful East Berlin escapees remained in West Berlin at least for some time
Their identities became known to the Stasi very soon after the escaoe
The city-state of West Berlin was teeming with Stasi informants
why did the Stasu not assassinate or kidnap the escapees after the fact, on West Berlin soil? This could have been the ultimate deterrent: "So few do escape, and even if you do we will hunt you down and you will die or be taken back East to rot in prisin anyway". All hope lost, and wasn't Stasi all about depriving people of hope?
They definitely had the tintelligence and technical means to do it, and I would not put past them having strong motivation to do it. Why was it not done?