We have no pets and a toddler in nappies, so there's no obvious reason that there should be actual pee on the floor.
I noticed it when I got home from work, it was quite strong (I've got used to it now). It's largely around the front door, front hall, front room and back hall (imagine a long thin Victorian house, so all the rooms run off the front and back halls, which join in the middle by the utility room).
It's not in the bedrooms, bathroom or kitchen.
That doesn't make a lot of sense as there's a bedroom between the front and back halls, so I'd have thought it should smell pee-y too, but no.
It smells a bit like a mouse cage, slightly sawdusty and a definite whiff of urine. But I can't see any evidence of mice.
Do you have any ideas?