then if you want you could put two env_renders in to make the locked door dissapear, and the door that can be opened appear. So when you find the key you trigger the renders and a text. (Text stating you've found the key), then on the other side of the door make another trigger a text stating "you've used the key. The door will remain unlocked"
It makes the attributes of the game much more fun--plus no offence to the guy who wrote the tutorial, but the tut does'nt work--your missing a connected target field which screws it up and enables it to work.
I could figure it out but not any noobs... So ya azhra.. use this method for locked doors.