We've lived in this house for 6 months it is a 3 storey new build terraced house. At the top of the house is the main bedroom with en suite. The en-suite stinks of cat pee, not all the time but generally late evening.
I've scrubbed the shower cubicle, hand basin and the loo with every known cleaner. The vinyl floor covering gets washed with neat bleach but still the smell lingers!
Only asking because I know that mouse/rat urine has a very distinctive smell and it could be that there has been some "activity" under the floorboards/in the walls, in which case you will not be able to get the smell out.
If it is late evening then it is possibly because the heat from downstairs (cooking, lights on etc) reach their highest temperature at this time and thus makes the smell more obvious.
I have heard about light fittings smelling too, apparently it can be rank! The plastic fitting degrades and smells , its easily changed and they are only a couple of euro so maybe give it a go and see?
hot plastic electrical fittings usually have a fishy smell (and these days you should not get it unless there is a fault. Energy-saving lamps run much cooler than old filament bulbs did, and nothing else should get hot).