The back door wouldn't have had to be unlocked, according to many, many folks on the threads who reported that they were either shown how easy it was by someone, or did it themself when they locked themselves out. You can basically lift the panel of a sliding glass door up out of the track and the hook lock is essentially useless. People described the various enhancements they'd made to their doors to try and make them more secure - bolts, bars, etc.
No link, but I'm sure google will enlighten you. It's been exploited by burglars and rapists and killers for a very long time. I think the Golden State Killer liked houses with them, and was actually responsible for a home security education program for the police, telling homeowners how to stay safe. Fox in a henhouse, there. *shudders*
An additional anecdote, to illustrate how easy it is to get that panel out. No burglary, rape, or killing in this story. Just a housesitting mishap.
When my partner and I were young *sigh*... our very early twenties? We house sat for her parents who went on holiday. They had two big dogs - a lab cross staffy with all the height of a lab and the chest muscle and breadth of a staffy, and a lab/kelpie/shepherd mix of some kind who was found on the side of a road as a pup. I think we also had a big dog at that point, a standard poodle I inherited from my Nanna when she passed. Not frou frou - a ditzy, incredibly high energy nine year old who wore us out with regularity.
Back sliding glass door was older, quite a heavy one. We had it open about six or eight inches to let the dogs come and go potty.
There was a noise outside, something the dogs felt the need to immediately check on and bark at.
I don't know if it was the first or the second dog barrelling through the narrow gap who bumped the door from the track, but the following dog/s tipped it, and we watched, helpless, from across the room as the gigantic pane of glass fell in slow motion and smashed into a million pieces, covering the entire back deck with safety glass cubes.
I'll draw a veil over the next many hours of wrangling three large excited dogs, sweeping up buckets of glass, contacting inlaws on holiday (inevitably, they were slightly drunk, in the middle of the night - they were on holiday), and trying to contact insurance and a glazier to attend at the soonest possible time.
It didn't need special equipment, multiple people, or make a massive noise (not until it smashed). All it took a chunky lab staffy shoulder-charging one edge of the frame. A solid bump.