IMO, if the drow's strategy keeps working, there's no reason for them to stop using it.

I suggest keeping to the literary tradition of repetition. That being that when something happens a second time, you play it straight, but when it happens a third time, you subvert it.

Thus, on the third time your players experience this scenario, something unexpected happens that causes the truth to be revealed. Maybe they meet someone else who's found out the truth. Maybe they catch the villian in the act this time. Maybe they get lucky and find wherever the villian is stashing his supplies or minions and lay a trap to catch him there. The point is that either something goes wrong with the villain's plans or some outside force acts as a catalyst to trigger the heroes' discovery of the villain's plans.