Randall_Flagg
The Evil One
I've had this happen on various types of improvements. Went 50 turns not noticing an antiquity site 2 tiles from my capital hidden under an improvement.
Also, if you are just trying to deny the culture leader in a game access to artifacts you can turn them into landmarks with no diplo penalty to the civ you are digging in. Obviously you don't want to create landmarks in range of their cities, but its a great way to prevent them from stealing artifacts from other civs.
Oh what an awesome tip!! Had not even thought about that!!