Yes, like other said - just be vigilant in the training center or other places you suspect other dog have been marking. Usually they sniff first so if you see him even thinking about it, tell him no and give him something else to do (like you mentioned a sit stay). He'll figure it out. At this age he is still learning the rules and from his perspective, that spot is fair game as others have been using it. Eventually he'll learn not to mark indoors. I never minded mine marking outdoors (my intact girls will mark outdoors as well, but they squat to do it) but they also know the difference between an attentive "heel" and a neighborhood stroll. Marking is NOT okay while working but is while strolling.