Vanishing Rings

So, we (me and my fiance) moved into this flat back in 2010. It's a simple, 1 bedroom flat with a kitchen-lounge, bathroom and box room that we keep our birds in. We are in the upstairs flat and there is a neighbor downstairs who is a man who lives on his own with his dog. It was originally built as one house, but was converted into two flats when the local council got hold of it.

Not long after we first moved in, we started noticing the odd shuffling sound and about a week or so after moving in, I heard an old lady giggle right next to my head while I was sitting in the bedroom. We also see the odd child-sized black shape that peers round doorways and moves from the bedroom to the bathroom. This has also been seen by guests too. It's nothing all that bad at all and it never has been, but what has been confusing us is that all my fiance's rings vanish if he should place them on his bedside table.

It started off with a promising ring that I gave him when we first started going out. Now, he's careful with rings and had a habit of taking it off before going to bed and placing it on the bedside table. The night in question he did just that and woke up to an odd shuffling sound by the bed, fell back to sleep and the next day woke up to the ring being gone.

Then it happened again when I gave him a replacement ring about 2 years later. He placed it on the bedside table, we woke up to shuffling and it was gone again the next morning.

Last year he bought himself an expensive silver ring as a treat while we were on holiday and when we got back he started his old habit of putting it on the bedside table at night. And it happened again. Shuffling during the night and gone the next morning.

We've searched everywhere around the table to check that it hadn't just fallen off, but it's nowhere around there at all.

It's odd because we lost a metal bottle opener recently too and we woke up the next morning to find it in the middle of the kitchen floor. We wouldn't have minded that, but the rings are expensive and it's messing with our brains a bit.

Any thoughts?