It is possibly (probably) a clogged vent. Check the roof and run a snake through the vent in your bathroom down from the roof. Probably clogged with leaves and such.
Get yourself a blacklight and shine on your toilet area. You’ll find all the places you missed cleaning.
I have a hypothesis that the venn diagram of men who pee standing up and men who don’t clean the bathroom is a circle.
It could also be a unused sink/bathtub in the bathroom. My master bath has a separate tub and a additional sink, both that go unused for long periods of time. The water that is commonly in the trap slowly disappears, some say it evaporates but I blame the bathroom trolls, anyway, that allows the sewer gas to enter the bathroom, the simple fix is to run the water and fill the traps back up which I have to do every so often.
Though I suspect the carpet.
Heh. Back when we shared a bathroom I convinced my brother that washing the floor was his job thusly
him: Why do I need to mop the floor? I hate mopping, I don’t want to.
me: I can swear on a bible I’ve never peed on the floor, can you?
him: ::sighs and takes the mop::
Glue a small plastic fly or stick a silicon target to the inside back of the toilet. Boys can’t resist aiming for either of these.
Kym