There are many different door alarms based on motion, opening a lock, opening the door and so on. There are alarms that can even outsmart kids. Just need to look what the right alarm is for that door and that kid. Also if choosing an alarm instead of a lock, make sure it can be heard loudly where the parents sleep.

I wouldn't say it is bad parenting if a kid sneaks out. Children are often smart enough to get out but not smart enough to understand consequences. I'm surprised more kids don't go out like that, but when I was a kid I always slept through the night so it didn't occur to me you can do that. A kid I knew was just 1.5 when he got out and wandered a mile! He was safe but anything could have happened. Back then there were no fancy things so they had to put a padlock on the door but they said they felt unsafe with that like if there was a fire and smoke how could they find the lock?

It's good to understand that aspect of safety. Not all people do that. With kids there are always ways they can get in trouble, things one never dreamed of.