Here is my breakdown and theory from chapter 1 up to the latest chapter:
In the beginning, Luna was likely using hookup apps and already had an active casual partner she’d been seeing for a couple of months. That neatly explains the opened condom wrappers, the moaning sounds, and the scene where she was kissing someone early on—that was almost certainly FWB #1.
In Chapter 9, they had a complete falling out. When the MC got sick, she canceled her plans with FWB #1 to go take care of him. Even though she offered to make it up to him later, FWB #1 got pissed and told her to ignore the MC and come entertain him instead. Luna reminded him that they’ve only known each other for a few months, lost her temper, cursed him out, told him to never contact her again, and blocked him. That was the end of FWB #1.
After that chapter, there are zero scenes implying that anyone touched her. Any moaning sounds from that point forward were strictly from her masturbating.
Then around Chapters 15–17, Ryuji (her former part-time coworker) mentions past rumors from 2–5 years ago claiming she was promiscuous and slept around with guys.
After that, nothing suspicious happens until the current arc. Because FWB #1 was cut off and her libido is exceptionally high, she eventually relapsed and went back to the hookup apps to find a replacement—which the MC catches onto.
Prediction for the next chapter: I suspect the person showing up at her door is FWB #1. On the final page of the latest chapter, it looks like she is heading to the door to meet someone, likely FWB #1 coming over to make up, confront her, or try to reconnect.
Everything lines up and makes complete sense this way.
P.S. 1: FWB #1 probably hooked up with her at her apartment often in the past, so it wouldn't be surprising at all if he knows her address and shows up right at her front door.
P.S. 2: She is clearly very experienced and had plenty of casual partners beforehand, which is almost certainly where those workplace rumors came from in the first place.
The problem is that Luna isn't going to the front door, she's just going into one of the other rooms and nothing even suggests there is someone at the front door.