Well I'm on to something now...
I was just kidding about putting a lock on her harness but I think it might just be the perfect solution!
I searched the web and found this locking leash: http://www.goldielock.ca/
It seems this is the only company making these things which explains the high price of $80usd, and they're based in Canada. I'm not going to drop that much green on a steel cord and lock so I'm making my own. I'm thinking about using a thick gauge chain to wrap around both the neck and chest which will be secured at the top with a lock. From there I could use a good bicycle locking cord to secure her to something.
Great idea huh? This is going to cure all of the worries I've been having for the past six months! I'll post up the final result when finished so anyone else in the same boat can try it out.