How would you handle it? What about having strong suspicion vs having solid proof?

My mother and stepfather are both really immature in certain ways. My stepfather is a major flirt, and is also very needy when it comes to female attention. While I have never thought this makes him automatically cheater material, I have always thought his tendencies are red flags. Then there's my mother, who also easily feels insecure, but instead of acting outright needy, she masks it by trying to make others feel insecure, too.

So over the past several months, my mother has been making a point to work it into conversations with her husband that she shares sexually inappropriate jokes with her male boss and coworkers. Because of how well I know my mother, to me this is a blatant sign that my mother feels insecure in her marriage, and is trying to make her husband feel insecure, too, as a result.

Then over the past few months, whenever my mother is out of town (she is often out of town for days to a week at a time on business), my stepfather is out very late. Normally they are both home between 6-8PM on work nights. When my mother is out of town, my stepfather doesn't come home until very late, like 10PM or even much later, and recently he didn't come home at all, meaning he spent the night somewhere, but not at home. He didn't return until late the next night. When my mother is back home from a business trip, he goes back to coming home at normal hours after work.

So the situation is that I have a strong gut feeling that he is having an affair. So not only do I have no proof, just a gut feeling, but I'm not sure what the ethically right thing is to do about a situation like this. On one hand I feel that it's not my problem and none of my business. On the other hand, what if he gets my mother sick? My biological father cheated on my mother and have her an STI. Luckily it was an STI and not an STD, so she was able to have it taken care of at the doctor's. But what if she gets an STD? Will it have been morally wrong of me that I didn't say anything, that I could have prevented it, but didn't? Yet the feeling that I should mind my own business and not cause any drama is equally strong, especially since I have no proof.

PLUS I live with them both, and probably won't be able to have my own place for quite some time, due to a series of mental illness related events and current problems. If I did say anything to my mother about what I've been observing, I would have deal with any fallout and blowback 24/7.

What are your thoughts on a situation like this?