IME you only ever stay out and sleepover because you’ve met someone as are guaranteed to get laid
I would assume not coming home meant having spent the night with another woman. And I would assume that even more so if it's a one off.
Why are you with someone if you can't even trust they are telling the truth about missing their bus/train as a one off?
Unless they were showing some other signs of being unfaithful or it was a regular thing, if my partner ever said "Sorry missed the last train home, crashing with a friend, be back in the morning! x" then my first instinct would be to trust they were being honest and think they were an idiot for missing the last bus/train not assuming they were cheating!
Being cynical, I'd double check when the last train was. Just after 11pm seems early for the trains to have stopped, unless there was an incident. Could he not have got a taxi or a bus?
Not everyone lives in a city where post-11pm public transport is standard. I live in a rural town, the last train to the next town over on Friday and Saturday is at 23:27 but the last train from the other town to here is 22:42 ... so if we go out in my town my friends can get home after 11pm, but if we go out in my friend's town then I either have to be at the station for the last train, spend £35 on a taxi, or crash on my friend's sofa. If it's a weekend then there is one last bus back to my town at 23:58 but it takes the country lanes - so it takes over an hour for what is usually a 10/15 minute journey on the direct route via A roads - and terminates at the town centre which is the closest stop to my house but still means 20 minute walk, alone, through town at 1am (when all the pubs/clubs close so taxis are scarce)
If we're just having a "couple of drinks after work and a catch up" kind of night then getting the last train is rarely a problem but sometimes we have been sitting having post-dinner drinks and I've suddenly realised it's 22:30 and I'm not going to make the last train home as it leaves in 12 minutes and I'm 20 minutes away. If there was something important I had to be home for (eg it was my week with DD and I had a babysitter looking after her) or I had an appointment the following morning then I'd suck up the £35 fare and order a taxi (which would still usually require a wait at that time) or risk the bus gauntlet. If I didn't have to be home, I'd crash with a friend and it would be nothing more than the most convenient option.