A question that came up last game; Does a Tactical Retreat trigger PDS fire? The text for TRs does use the word "activate", but retreating to another hex that's already activated certainly doesn't trigger PDS fire. I see three possibilities, listed in order of what I think is the likelihood they're correct;
1) PDSs do not fire at all in this circumstance. 2) PDSs fire as soon as the adjacent hex is activated (so, before the combat is actually resolved; the retreating fleet won't be targeted.) 3) PDSs fire once the fleet leaves the system.
It wasn't actually important in-game (No one had DSC), but it was something we could see coming up in the future.