Right, you buggers, I back with my update and I am pleased to say.... drumroll.... MYSTERY IS SOLVED!!
After delivering the 24hrs notice to come in and inspect notice under the door yesterday, also sending the same message on Facebook (couldn't find her on any other social media) and trying numerous times to reach her on the phone, my daughter and I went up to the flat, unlocked it with my key, aaaaaaand.....
It's immaculately clean and tidy and no-one was on there. Both of my notes were lying on the floor by the door, untouched.
On closer inspection, we could see there was a pile of post that has been gathered up and left neatly on the table, and that there was a thin layer of dust on everything, including the hoover, so it obviously hasn't been cleaned up recently, but left like that. All the electrical appliances, including fridge were switched off. Gas cooker disconnected oops, sorry, electrical cooker also switched off.
There is no evidence of anyone living there except my tenant. There was an open clothes rail of female only clothes, no male grooming products around, a single pillow in the middle of her double bed, photos around with no partner in them, but with her in them.
However, there were a lot of beautiful houseplants around which were clearly very well cared for, not dying, and when we touched the soil, could see it wasn't dry.
So there you have it - She's gone away somewhere, is clearly intending to return and someone has been popping in to water her plants. That will be whoever answered the buzzer!!
We left, satisfied the mystery is solved and all is well.
But then we got back downstairs to my flat, and we decided that it would be sensible to leave a third note by the plants to ask whoever has been watering them to please give me a quick ring, just to confirm all is well.
I sent daughter up with the note and she didn't come back down for ages, so I went up to investigate. I found she had been accosted by the lovely (male!!) owner of the flat next door who had heard voices and someone going in and out of tenant's flat, thought tenant had returned, and had got daughter chatting and was able to confirm that it's him that's been popping in to water her plants and pick up the post a couple of times a week, and that tenant has gone travelling around Asia for around six months, but he's expecting her back this month, and that she often doesn't have phone signal or internet while travelling. This is entirely fits with my lovely tenant's freestyle hippy personality and is a completely believable story.
So there you have it mumsnetters. Sorry it's not some dark secret and has a perfectly innocent explanation that is probably rather disappointing and boring to you lot, but a huge relief to me.
Keep being amazing - and remember, do your gas safety checks!