Calling it now, we're building up to Saitou eventually giving Yokoi a pair of her used, possibly farted-in panties. It's not just a gag.
This chapter and the chapter that this is a counterpart to, Saitou-san Observation Diary, shows us Yokoi's desire to understand and give a voice to people who have complicated feelings that they can't fully express. This has been very consistent with the way he lives with Saitou - when she has a fit or hurts herself or reacts in an uncomfortable way, Yokoi either knows how to help because he's been learning about it, or he doesn't know and he clearly tries to learn about what's wrong so he can help now and in the future. This isn't reciprocated by Saitou. She starts the chapter wanting to understand him, pushes him too far by lying to him about Ju-On and ends it by rejecting something personal about him. There's an unhealthy imbalance. Her observation diary ends in failure.
I believe her conclusion will eventually be that even if she finds Yokoi's smell fetish too weird, she can accept it and help him in her own way. She's already wiped her face with his shorts multiple times when he wasn't around and expressed that she likes his smell, so she can empathise with the concept on some level. They'll be panties because they're more intimate than socks, Yokoi would see it as crossing a line so he wouldn't have sniffed them before. Saitou originally showered 5 times a day because she couldn't relax when she was dirty. In Saitou-san Observation Diary, she dropped down to 3 times a day. She started showering more pre-AC, and then she said she'd shower less overall if they got an AC unit, so right now she's showering less than normal and she's okay with it. She could easily shower once or not shower at all without Yokoi finding it odd, giving her panties ample time to collect smells before giving them to him. Farting in them would close the loop on his desire to smell a fart in a bag - she already tried it and it didn't work, but she knows that cloth can retain a strong smell for a long time.