Ah spot checks. The best thing you can ask for to get your players paranoid. Also the best way to keep them off their guard if you ask for enough of the. "Oh he wouldn't have something here, we've been making spot checks for the past ten minutes and it's always been nothing". Then they fail the critical one and surprise round occurs. Even better if it's something that they could have noticed. Though this brings up my wish for degrees of success and failure in more mainstream RPGs. In the poster case you could have them notice the feet, but not describe them as feet. More perceptive characters would actually notice the toes and be able to deduce that something may be exploding out of the ground in short order.