So everything was still latched up and the bottom tray still inside when you got home? Amazing. I find that most dogs that get out do so by knocking the bottom tray out of the crate, turning the crate over, and squeezing through the wires at the bottom of the crate which are usually spaced further apart. Yours doesn't look like the kind where that's possible, though, so he probably squeezed through the door…
Paco always tries to escape, especially since two or three times, we didn't realize that the kids next door had unlatched the side door of the crate while playing with it, so he has managed to escape a couple of times. We'd come home and there he'd be, waiting for us at the front door.
Once he realizes he can't escape, though, he settles down and naps until we get home. He refuses any special treats or toys we give him to occupy his time; instead, he just sleeps. I know because I've left a camera up. Perhaps you should try doing the same thing to give you a little peace of mind.
The times where Paco's escaped from the crate, he's never made any trouble. The time where we decided to "experiment" with leaving him out of the crate, it was a disaster. I think the "escape" gives him something to do , and then he just wants to wait for us.