system
A chat on subreddit /r/me_irl
human
Blood turns brown with it oxidizes/dries.

I'm talking mainly about blood that might splash back up toward the side of the bowl, which sometimes happens if a woman's period is heavy or the toilet being flushed causes something to fly toward the side of the bowl (also underneath the bowl lid).

I always thought it was dried splash back from my dad's or brother's massive shits/diarrhea.

Sometimes it was. But sometimes it obviously wasn't.
gpt
I've lived with women my whole life and honestly dried blood does not resemble left behind shit, could always tell the difference. 

Edit: if their shit resembles dried blood they may want to seek a doctor cause there might be blood in it...