They might be looking at your iMessages of your unlocked phone or your gmail account on the computer that you left yourself logged in on.
Again, the weakest point of any security is the human in the loop (read: you) that doesn't believe it can happen to them, regardless of how your data is accessed.
I am looking forward to a wider release of google glass for reasons like this. I can't easily jump into my pocket and pull out my phone to film a cop without, probably, getting shot but to be able to have it ready to go with some sort of motion/voice activation... will be so great.
This is great security for what it is. Probably enough to keep you 98% secure.
Which is still exactly 0% secure as far as I'm concerned.
All in all though - in general - I'll be more than happy to continue using iMessage and feel at peace. As a general rule, however, never send anything electronically that may screw you over later.