He might recognize exactly what it is, but the smell would be out of place and that might get him suspicious. The transmitter for the detonator might make a beep every once in awhile.

Another thing, the bombs wont just go off if they're tampered with. C-4 takes a combination of extreme heat and a shockwave to go off. So if the walls were damaged in any way, the Fairy Tail people would be able to find them without them going off.