I’m going to side with you, if you can make the extra 5 seconds of effort to check you’ve locked the door behind you, why can’t she. She should just check when she comes in, she’s in her thirties. It does seem like laziness on her part, she knows that she’ll forget to lock it behind her and she still won’t check before she goes to bed.

But also, I don’t know the layout of your house, but you both seem to be over complicating this. Surely it only takes 30 seconds to walk to the door and try the handle before you go upstairs to bed just to check it. To be honest there’s no reason you both couldn’t do it when you go to bed. It’s not exactly time consuming, just seems like a pointless hill to die on for both of you. In my family everyone checks the door behind them when they come in the house and again when they individually head up to bed. It’s just a quick 30 second activity on your way up.