Slowly marking his place to ward off potential rivals.
I interpreted this as
her adding more things. Because he's the one that remarks about there being more things (he says "at your place" and they're at her place since he's the one to say he'll come back the next day) and she acts startled and tries to dodge the issue. It's almost like she's prepping for him to be able to stay the night, but can't bring herself to ask directly.
EDIT: wait, I'm an idiot. She says
all the dialogue. So it is his place and he is adding more things because the line about returning is her calling him senpai.
I think what confused me is the last line of "maybe it's just your imagination" which feels more like a reply to her pointing out the increase in things rather than a continuation of the thought. Is that a mistranslation? Like maybe it's supposed to be "maybe it's
my imagination" and this is just another translation subject/pronoun trap because Japanese is weird like that.