Its a nearly impossible long shot, but you could ask if they have surveillance video of their shop dating back that far. You can look to see if some moron wiped brake fluid off his hands with a rag and set it on the hood. Alternatively, see if there is a test that can be performed to identify brake fluid in the paint? Also a terribly long shot.

When it comes right down to it, talk is cheap. Walk in there politely and ask the service manager what he thinks of the damage and explain that it started after the service and you JUST figured out that it might be brake fluid. The worst they can do is tell you to kick sand. Under some miracle, they might say "yeah, ol' Jeff was an idiot and did that to 5 vehicles before we fired him..."

Another alternative? Find a sticker or vinyl wrap that looks good in that spot, clean it well and slap it on there! Cheesy, but effective!

Best of luck and I hope you find a good solution.