Cool game! I like the MacGuffin idea. The art is neat, but rocks look like physical objects. I tried to hide behind them and "died". Also, sometimes I couldn't tell if I'm camouflaged or not. Perhaps the default player's sprite should stand out more.
I was a bit disappointed that the terrain you're standing on doesn't matter. I can camouflage as sand and be invisible to the guards while standing on the grass. Otherwise, it's a pretty good concept :D