Ah what the hell, I'll share. So my last ex gave off a few as the honeymoon phase was ending that hinted at a much bigger problem. I'd say a total of 4. Maybe 5.
The first one was her inviting me to go with her and her friends to Vegas. After I told her I'd go with her (I think it was 1-2 days after) she uninvited me. This wasn't too big a deal. She was an independent person who liked her space and alone time. I respected her wishes and gave her any space she wanted. Plus she did go to Vegas with me later. I still thought it was a little weird.
The second was a slight miscommunication. She had been living at her parents' place while she was saving up to get her own. She and I had talked about the idea that one day she and I would live together and I'd move in. She thought I meant as soon as she got her own place when she wanted that to happen a year later, maybe a year and a half later. I don't know what signals I was giving off that made her think that, but when we talked about it, I clarified that I meant a year and a half from now. I had my own life to work on as well. She was glad to hear that.
The third was that she wanted an open relationship. I know, big warning sign for a guy who has never been in one and doesn't want to share his girlfriend. However, she wasn't going to be with anyone else and wanted me to be the one seeing other women. She was okay with it as long as I spent equal time with her and any other girls. I entertained the thought for a few days but never acted on it. I asked her again if this was what she really wanted. She told me it wasn't and that she said because she thought I was getting too attached and wanted me to back off emotionally. She figured seeing other people would help with that. I guess that could count as two since it prompted me to ask her if she'd ever cheated before and the way she talked about it was nonchalant, as though it was just something she tried on a whim.
The fourth (or fifth) flag was what led me to break up with her (although I'd say we both knew it was over when we broke up). She told me she planned on leaving the US to go get her Masters degree in another country. I of course wasn't going to stop her from doing that, but as she told me about this plan, I realized I wasn't a part of it at all. The fact that I was just finding out about this after she had been wanting space from me was also troubling. For me personally, this red flag comes with baggage, since the first girl I have ever loved left US while we were together (for different reasons) and what followed was a year and a half of emotional hell.
All of this led to me confronting her about breaking up. We both knew it was the end and while we both cried at the time, I see now that just like her cheating, I was nothing more than an experience to her.
TL:DR My last ex gave off 4 or 5 red flags just as the honeymoon stage was ending. They all pointed to her not really caring about me at all and being selfish in relationships.