There are certain keys in the game (anyone have a list?), such as the Trak Idol or the HS Key, that when used once will automatically put a 'copy' of itself on your keyring, which does not take up an inventory slot itself. Keys on this Keyring are not tangible (in game sense). You can see if you have any keys on the ring by typing /keys in game. With the keyring, you do not need to have the key on your person to open a lock, just click U and the door will open, as long as you have opened the same lock once before with a tangible key.