@LoveFall
It seems to me very unlikely they would have taken it as it would be so obvious.
Something similar happened to me. We went away for three weeks and I did not take my engagement ring. I took it off and put it inside a sock in my sock drawer. When I came back I did not remember putting it there, and searched all the other places I had hidden things. I was convinced it had been stolen. I claimed on insurance and everything.
Just before I got my new ring I moved something in my sock drawer and it jingled. Yup. My ring and another piece of jewelry in the sock. I cancelled the insurance claim. I think they thought I was crazy. No, I didn't have dementia. I'll bet you put it somewhere safe.
This. ^
Chances are high @StarWa that you - or someone in your household/someone who's visited has moved it/put it away... in a drawer, on a shelf, under a cupboard or bed etc.
As pps have said, it's extremely unlikely that this neighbour has stolen it! She would be the most obvious 'suspect' as she had your key!
One time (some 10 years ago,) a work colleague (Carol) gave me her debit card, and asked me to get £50 out for her on my lunchbreak when I walked into town. She said '9293 is the pin.' I was not keen, but did so anyway.
About a month later, on a Monday morning, another colleague (Lyn) took me aside, and said 'I have to tell you this Frosty; Carol's had £60 taken from her account, from the Barclays at Gann Street. It was lunchtime the day before yesterday whilst she and you and all of us were here at work. She says only you know the PIN. Not even her bloke knows it. So it must be you. She is thinking of ringing the police.'
I was gobsmacked and actually really upset and angry. I said 'I ain't bloody had it.' She said 'well she's left early today, but is in tomorrow, you need to mention it to her.' I told our manager and said 'she's threatening to ring the police (101.) So to hell with it, I will ring them.. She seems keen to blame ME for the theft, let them deal with it.' The manager said 'yes OK! Let's do it.'
The police came the next morning, and called her from her desk to give a statement. She went really red and looked uncomfortable. Accuse someone of theft? Deal with the police. It's a police matter IMO.
Long story short, the police said they are going to obtain the cctv footage from the time of the withdrawal as the ATM has a built-in camera. Suddenly she remembered she had withdrawn the money on the Saturday morning when she was in town, to buy some electric for her key, and some wine, and had 'forgotten' about it.
I don't know whether she genuinely forgot, or if it was some attention seeking drama shit, or an attempt to ostracize me, or what, but I was livid, and couldn't be arsed to speak to her again. She never even said sorry! A few people turned against her after that, and only spoke to her if they had to. She left about 7 months later.
SO, think very carefully before you accuse people!