Thanks for the ideas everyone - Joeve, that's what's driving me crazy lol. I have moved the couch around so many times cos I swear that's the most logical spot lol.

I've just emptied the freezer, no luck I got excited about the brown drawer suggestion, there are brown wooden drawers in the kitchen I hadn't checked, but it wasn't there. The display cabinet by the door is also wooden but doesn't have drawers - I still looked again though lol. And checked the desk drawers in the spare room cos I cleaned it the next morning, but still nothing.

I'm having the bad thought that it may have gone in a Salvos bin... There were three bags of old clothes by the cabinet on Thursday night, and on Saturday morning I put them in the Salvos bin. This was before I realised the key was missing. Big bugger if it's gone in there!