Haj was a great guess too

What if the concern wasn't that he would have to use a disgusting bathroom, but rather that he takes horrible shits and didn't want someone to know?

I mean, I'd certainly be embarrassed to have to tell the internet something like "I'm visiting my in-laws for a few days, and I really want them to like me. The problem is that I shit like a farm animal, and they only have one bathroom."

My guess was cave exploration. He said he was going to a remote area where pooping was an option, but he didn't want to, which makes sense when you know you've gotta carry that shit out with you in your cave pack. There are only a handful of scientific cave expeditions that go into caves for three days, so I figured that if he said so, he would unequivocally dox himself.