Hi Titchmouse! Is there any way that you can put a mirror in front of you when you're at the computer, so that you can see who's in back of you, and anyone who comes through the door?
It doesn't have to be an actual vanity-type mirror; if you can find a small "garden ball" (those mirror'd balls on pedestals, that people put in their gardens) that will work. Or, create some art that incorporates bits of mirrors, or a few convex mirrors (sold for about $1 in the automotive section at Wal-Mart, etc), that can also serve the purpose.
The first thing to do is to make it so that you never feel as if someone could enter the room (or workstation) without you knowing about it. A mirror is the usual solution for this.