IMHO — it's worth introducing into the HeroesWorld rules (or as a sub-clause of clarification). Otherwise, the current practice is: a player intending to pass by a guard and assuming the opponent has been there before asks the opponent a direct question — "can I pass here?". The opponent is often forced to provide important reconnaissance information... there's room for speculation, by the way...
Added after 9 hours 48 minutes
Maybe it's worth adopting it? With the sole caveat — "the concept of redrash is not applicable if both heroes end up on opposite sides of the subterranean gates (portal) during the attack"?