Hi Adrian,
Ouch, if you'd been just one model of iPhone later, the back glass would have been replaceable without a laser machine. Rats.
Anyway, I ran across this guide a couple of weeks ago in the course of answering another question, and I'm thinking this might be something to try. It's a guide to applying oleophobic coating on glass; that's the coating that keeps fingerprints from showing up and helps repel liquids. I'm thinking perhaps your case wore down the coating on that one spot, so perhaps renewing the coating would fix the issue for you.
[guide|9682|How to Restore or Apply an Oleophobic Coating - iFixit Repair Guide|new_window=true]
There's a link in the guide to an Amazon vendor selling the liquid oleophobic coating to use.
Good luck! Let us know how it works out; I'm curious to find out if that's the problem and if the recoating will fix it.