The trap wasn't making much of an effort to hide itself, once he could peer beyond the wards; it was standing there, like a hellhound on a leash, snapping and barking and waiting for him to get just close enough. It certainly wasn't enough to kill him outright, but walking into it would be uncomfortable, to say the least, and quite deadly to most mortals. It was very clearly meant more to discourage people from poking their noses in, than to actually kill, as anyone who could find it would see the trap, very obviously, as well.