I'd use hiding rules. In AD&amp;D that would be a hide in shadows roll for the thief to not be in line of sight, in d20, anybody could attempt a sneak check, or perhaps an acrobatics or tumbling check to see if they can get behind cover of some sort, and appropriately hide themselves.

Without a map or miniatures, how to best determine line-of-sight, etc?
I tried to google it, but didn't quite get it. Any help and/or examples?
Thank you!
