Something similar-ish happened to us some 30 years ago. Me and DH moved into a little rented council flat (our first home,) and DH was at work and I was off for a few days, sorting the flat as we moved in just 2 weeks earlier, and hadn't had much time to sort anything. I had the chain on the door. (Thankfully!)
I was in the bedroom sorting the drawers, and heard 'clank bang rattle rattle.' I shot into the hallway, and went to the door that lead into the flat, and a woman was trying to gain access to the flat. She had a key and was letting herself in, but she couldn't get in, as the door only opened 4 inches or so, because of the chain on the door!
I said 'excuse me! Who are you and why are you trying to get into my home?'
She said 'my daughter Gail lived here for 10 years, and I was coming back to take a last look at before anyone else moved in.' For context GAIL left around 2 months earlier, so why she left it so long just baffles me. She (Gail!) also left it a fucking shithole. The council took 5-6 weeks to clean it and remove a load of rubbish she left, and do some repairs!
I said 'well it's not vacant! Me and my fiance have moved in!' Long story short, she wanted to come in and look around, and I said no. She seemed a bit aggrieved. FFS the woman's daughter had only moved out less than 2 months ago, and she would have seen her for the 10 years she was there! The woman started crying and said 'she had many happy years in here, it's so sad she had to leave. But there wasn't enough room for her and her new fella and all his stuff too...'
I didn't give a shit and just wanted the key, but she wouldn't give it to me, as her daughter had had it cut and it belonged to her.
I said 'I am going now' and shut the door and bolted it from the inside. I rang the council, and explained what happened and they came and changed the lock.
Silly bloody woman!
And what a nerve just letting herself in, when the flat hadn't been occupied by her daughter for 2 months!