It's not a modern problem. You could always rig a tamper-proof device (eg booby trap) to destroy evidence if forced without the key.
If you stab people, you can wipe the blood off the knife. If you shoot someone, there was not a way to associate a projectile with a firearm. (Assuming the dubious merits of "firearm forensics".) Back in the good ol' 1780s, there's no presecutorial evidence to be gained by inspecting my safe because you cannot show that the knife/gun inside was the murder weapon.