It's alright scoffing and laughing but there are some thing guys would rather not know about and this is one of them. Keep a diary and if it happens every 4 weeks you know the answer. Unless you've more than one female in the house.
Take it from one who knows.
Check for any leaks. Poor aimimg as mentioned is a sensible answer although normally smells of urine rather than fish. Silly question but are you sure the smell is definately coming from the toilet or could it be from an electrical fitting ie: light switch/fitting/downlighter?
Also just a thought, some makes of toilet seat tend to hold odours more than other makes. If it is a rough porous surface on the under side of the seat, this could need a scrub. I know you clean your toilet but sometimes we ignore that side of the seat.
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