"There's a whiff in my cupboard, dear grandma's dear grandma's...........

Not entirely sure ,Anno probably solid, but it's not that sort of smell!

Is it a floor cupboard or a wall cupboard?

Do you think a cat brought in a mouse which escaped? we have a piece cut out of the bottom board underneath the cooker and once had a hamster climb behind the cupboards, there is a gap between cupboards and wall. Luckily we managed to get it out without removing all the worktop and cupboards and before it gnawed through them all. Perhaps the mouse went in there and couldn't find its way out?
Can you take the back off the cupboard and have a look with a torch?

Otherwise wash it all out with a solution of Milton?

Do you have floorboards or a solid concrete floor? Something might have died under the floor.

Doesn't seem to be kitty I've checked every item and they seem to be whiff free, it is really mysterious AND annoying!

Is it the tupperware?

The house was built in 2005, block and brick construction, kitchen units from somewhere like Wickes, B&Q etc, the builder put them in.

Is it a wooden-lined cupboard built into the wall (what is called a press in Scotland)? Does it have a wooden back against a wall?

Perhaps the back of the wood has mould that can't be seen from your side?

See whether the point of a knife sinks into the wood too easily. If it does, get it looked at. I don't want to worry you () but dry rot can creep into woodwork invisibly. It gets its moisture from long tendrils stretching back to dampness elsewhere, that is why it is called dry rot, it doesn't seem damp itself. It smells a bit like you describe.

sorry !