Hmmm, the WMUR articles blur the info, making it unclear when her mother discovered the doors. It seems to me as though the mother's boyfriend peeked in Laureen's room, and thought it was her in bed when they found the front door unlocked. So maybe the initial reaction was the front door wasn't locked properly, they checked on her, she was fine, and then they noticed other things that were weird--the lightbulbs missing, the pillow and blanket on the couch, maybe some evidence of drinking or that another person(s) had been in the apartment and went to talk to Laureen and realized it was not Laureen after all. Or maybe the friend woke up to pee or get a drink, etc. ?
There has to be some explanation for the gap of time between coming in around 1:15am and reporting Laureen missing at 3:45am.
I rented a 3rd floor apartment once in a different NH town when I was really young and my back stair case was indoors. It was smaller, and I didn't typically use it, but it was a normal staircase. Think of an old Victorian style house, made into apartments. It went down to each level, and there was a door that locked at the bottom. There was also an attached barn type of building, but I didn't have access.
This was a slightly smaller town than Manchester, so it is hard to know what the back staircase in the building Laureen lived in was like. I have heard the building described as a "triple decker", and that kind of apartment building could have had both a second interior stairwell, and a fire escape.