Once he inspects the door more closely, Draco comes to understand the lock a bit more closely. It is a well made lock for a secure facility, not like the ones in the sewers or at the prison which nobody would have the means to pick the locks other than guards. In addition, there are extra mechanisms installed to trigger should any tampering happen: a trap.
He doesn't know what effect the trap will have, but he can judge that if it is just the lock, it will be small.
Oh, and the hinges are trapped too. Had Tiberius succeeded in breaking the door off with the trigger still active, it would have broken the pipes inside the hinges and released a non-solid substance.