* Sage, in the presence of Maya and Josy, noticed that the girl dressed as a mummy was Bella.
* Josy expresses surprise when discovering that the MC already knew, even before them, that the girl dressed as a mummy was Bella;
* After Josy asks if the MC knew it was Bella, Maya laughs and then says: "Haha! Of course, he did!" Maya's attitude was quite suspicious, it's likely that she already has her guesses about the MC and Bella. Which doesn't sound far-fetched, depending on the player's choices and Derek's loose tongue. Maya can even make future comments based on variables created by the DPC (for example, staying at Bella's house, receiving help from Bella to clean the MC's room).
MC is a party planner and responsible (or co-responsible) for guests list. On Bella's branch - MC invites her, Jill doesn't communicate with both of them (MC & Bella) for obvious reason and MC is able to easily admit to anybody that he invited her, except a few persons like Quinn and maybe Derek, according how he behaved in front of her. While when MC on another branch, then Jill invites her and MC recognizes her only during drinking game and asks who invited her in front of J&M and Sage.
* At no point is Bella seen with Jill at the party, if Josy mentions to Jill that she didn't know how much fun Bella is and decides to thank Jill for inviting her... Things go downhill. Even more so if Jill says that she didn't invite Bella to the party (or mentions that things aren't right between them).
When MC is not on Bella branch, Josy doesn't speak with Jill about Bella and how fun she is. Otherwise Bella tells Josy that Jill invited her (in front of all, including MC). Jill will never mention and talk about MC & Bella relationship to Josy. This may happen, if Josy will aware, especially if MC break up with throuple, and then she may ask Jill about Bella, because she know that they are friends. But it's a speculations, we can't predict how DPC will play out all this branches, break ups and their outcomes. While Jill is upset or even mad on Bella, she is not that type of person to get a revenge or whatever else in a way by making such delicate info a public state.
* Sage is not stupid. If she notices anything between the MC and Bella she will mention it between the lines the next time Bella is helping her with her studies. I doubt Sage won't mention how much fun she and Bella had together at the party, it's the perfect chance for her to ask whatever question she wants or make some silly joke if she's noticed something between the MC and Bella.
Yeah, and she covers Bella & MC ass at the best costume competition. I think she is one of a few persons whom will be ok to find out and accept the fact that MC & Bella in relationships.
* The scene of the MC and Bella on the dance floor is a real shot in the foot for Bella. This scene MUST have been on purpose, Jill definitely wasn't the only person who saw the MC and Bella dancing together. And those who saw the two dancing realized that they were a couple: they danced looking deep into each other's eyes, slowly, while the girl dressed as a mummy put her arms around the MC's neck. This is not a dance between friends or teacher and student (if Sage, Maya or Josy saw them dancing).
We don't have any evidences that somebody recognized her. Only Quinn suspected something and was interested with mystic "mummy" too much. Even Derek hadn't recognized her.
Following this logic - each dance had to have consequences. J&M dancing with MC, and Tommy should show in no time, Jill dancing with MC - Sage should knew that since there a lot of dormies, which know Jill, even if Sage was in another place. Dance with Sage looks as the safest option so far.
* Derek almost recognized Bella's voice at the party, during class, he could tell that Bella's voice sounds familiar and remembered the girl dressed as a mummy. It's enough for Derek to find out about MC and Bella;
He was too drunk and blacked out at the end. He will remember something of course. But it's a secondary problem, since Nick seen her coming out of MC room. Derek fucked up a lot, telling her about his tricks during classes
It's likely that the members are already waiting for the girl dressed as a mummy to come out of the MC's room. It's been shown that when it comes to the MC with girls they are very curious and perceptive, like the time Leon noticed that Bella hadn't left and had spent the night with the MC or when Jacob realized that the MC took a much larger than usual amount of breakfast to his room (because Josy was shy/scared to have breakfast at the table with the others). So if a rumor breaks out between the MC and Bella or a rumor that Bella was the girl dressed as the mummy... BOOM! They also noticed that there is something serious between them.
Last time it happened when mansion was trashed, all DIKs was in place and sober. It wasn't a party or something similar. So, in such case you are getting great capability to notice something, to wait for something, etc. During the party... especially so huge, with a crowds of guests, when most of them in costumes, etc. It''s hard to remember something, to notice something, etc. But knowing about the "tradition" he asked her (and she was awared already, because she did the walk of shame before) to come out through the window. And Bella noticed, that this party is the only one, she is able to visit and participate in, because all people in costumes and she can hide her face and body. And she had doubt a lot - to come or not to come.
There are a lot of plot holes if the DPC chooses to keep the MC and Bella's relationship a secret or not create any rumors about the girl dressed as a mummy. Let's see how DPC will handle this.
Relationships with Bella is the most problematic by a lot of means. It's her past, her husband, age gap, her work at B&R, Stephen, Quinn (if she will find out) and a lot of people around, which mostly will not happy with this.
But, tbh, any of MGs have their own troubles, too:
J&M - the throuple thing itself, J&M lesbian thing, Patrick, Tommy and maybe Quinn (if and when she will find out about it);
Jill - her family & all possible connections to MC family, Bianca, Tybalt;
Sage - her family (Stephan and Tybalt too), Quinn - in every sense.