When Microsoft announced at E3 that you’d soon be able to buy, earn, and win props for your Xbox Live Avatar, they also disclosed that there was a terminal tucked away somewhere in their booth where you could sign yourself up for an exclusive E3 trophy.
It wasn’t really “hidden” per se, but it was absurdly hard to find. It took me 4 passes and a few stops at the front desk to find someone who had any idea what the hell I was talking about.
If you were at E3 and managed to find the terminal, check your inbox. The trophies have now started to trickle out. If you weren’t or you didn’t, you’re out of luck; like most unlockable content, each E3 trophy is tied to a unique 25 character code.