My daughter bought a house and this thing is stuck on the front of one of the toilet tanks. What the heck is it? Seems to be placed there by the previous owner rather than being part of the tank.
Does it have any type of grooves/connector on the inside? It could be designed to hold a removeable air freshener module.
I know exactly what it is, because there’s one on my toilet, and I put it there. It is part of a childproofing lock for a toilet. An arm glues to the top of the seat lid, and then snaps under that piece on the tank. Supposedly it keeps babies out of the toilet.
If there isn’t the matching piece on the lid, then either the toilet seat was replaced, or they bothered to take that part off. The arm on mine broke long ago, and I’ve never tried to take it off, so no idea how easy or hard that is.
Makes sense, they had one or two young children. They could have removed the other piece when they sold the house or removed it when the kids were no longer babies.
When our kids were babies we childproofed a lot of things but never thought about the toilets.
This raises a whole debate about how much childproofing we should do. Maybe another thread though.
a little off topic, but there’s an entire forum on reddit dedicated to questions like the OP.
It’s a fun way to waste time…Look at oddities and see if you can figure out what they are.
And you learn stuff, too.
hey…It’s more interesting than playing Wordle. 
For example, this one from yesterday:
https://old.reddit.com/r/whatisthisthing/comments/u20fbr/green_circular_tool_with_spikes_we_found_this_at/
Solution: large animal castration device
After years of opening child-safety latches on our lower kitchen and bathroom cabinets I finally removed them.
I think my youngest was about 12, then…
Just about when I put locks on the liquor cabinet…
…
QFT