I have searched the rulebook and there is no answer to this question. I have no idea why these circles exist, but we play that a guard CANNOT stop their movement here. This would make it almost impossible to accomplish a successful escape attempt - the guard would simply have to stay on that space and the Escape Officer would do all that work for nothing. It's hard enough to make an escape happen - this would make it almost impossible.
Why do you play that a guard cannot stop on those circles, as the rules DO allow a guard to do exactly that? It's very possible for an escape route to go through that circle despite the presence of a guard... that's why the "Force Arrest" rule exists!
Escape from Colditz requires teamwork. Have one POW "take one for the team" by entering that guard's spot to force an arrest, thereby sending the guard back to the CC office, then the coast is clear for other POWs to use that route.