I don't like constant radius search because repeating scripts can lag if you have tons of doors...
Why not just use a trigger around the door?
I made this...I never released it because I figured it wasn't that useful. But if you would really like it, download it here: http://zerosan.dyndns.org/blrstuff/data/addons/tool_DoorCreater.zip (I gave to a few friends who wanted it)Basically, it does as MasterCE said: "Sounds like a constant radius search for players," which is exactly what it does. When someone is near, it's becomes transparent and walkthrough. When they leave it's normal. Uses the key model. (Note: It is missing a readme as I never planned to release it)