That's what made him fine, not the fact that you didn't lock.
That's a thing that quite misunderstood, but for a crime to be committed, you need criminal intent. If you just made a mistake and weren't aware it wasn't your house, it's not a crime to enter it.
Also for some crimes the intent needn't match the outcome. For example in the UK _Attempted Murder_ requires (as well as facts of an attempt) you had an intent to kill, but _Murder_ only requires that you had an intent to at least cause grave harm, the fact of death makes it murder, even if a jury believes you intended only, say, to hospitalize the victim.
But that's also the thing- we had no idea what his intent was. I do recall the police that night seeming less concerned because the door was unlocked and I was chided quite a bit. I do recall them telling me what I said in my initial post but I also understand criminal intent. Personally I'm glad the guy wasn't wrung up with a felony if he was just on some ambien sleepwalk bender or whatever he was on.
I'm curious what affect not locking your phone might have on police reports if your identity/etc is ever stolen and you need to provide police reports. Possibly nothing, but I can imagine some credit cards legal team working hard on that aspect of things in the event you're trying to convince AMEX to return $40k of transactions you supposedly didn't make yourself and need reversed because your phone was swiped while logged into some CC app and they copied your card number.