While working on my PhD I have diverged from the area of expertise of my supervisor and other people in the group. In particular that allowed me to find a near solution to a problem that was not solved for some time in the field where we all worked.
Towards the end of my PhD track I have started working on the draft for this, shared with supervisor and suggested that if they are interested, we can publish this together - even though all the work was done by me in this particular case, I still thought it would be a nice touch from my side, and also they might have given some interesting feedback e.g. regarding to connections to other literature. I did not really count on his technical feedback, as in all the humbleness I had quite some chances to see they are not very good at that - even coming to the point of making technical mistakes in the proofs, which were making the results wrong (in published papers).
So I have sent them several drafts of this paper, rather complete on last stages - just required writing some introduction and cleaning up here and there. They said that it may be a good idea to go with that. Yet, I have started working full time and as it happened, did not have enough time to complete this work (not much initiative was coming his side either). Imagine my surprise when a couple of years after I find out that the very same ideas were published in a journal paper by my supervisor and another person from a group where I have worked. No mentioning of my name anywhere (besides a link to one of my old papers, which dealt with a rather different problem).
Now, one of the things why I thought there was no rush in publishing my work is that it required me a lot of technical skills, which likely no many of the people working on that problem were equipped with - I am not sure how to make this sound less overconfident - but just let's say it was due to background, like you're having a physics degree coming to work on biology problems, and you're pretty much the only physicist in that little community. I was wrong in assuming that no one would attempt solving this problem, but I happened to be right in their technical skills: the proofs there are not correct at least in couple of places, as the authors (and the reviewers) overlooked issues there. Namely, similar results are correct under different assumptions, but not in the shape they are stated in this paper.
So what I think likely have happened is: my supervisor did not really plagiarize my drafts (otherwise I would have expected at least the proofs to be correct), but maybe looked at the ideas there, shared with my former colleagues and they redid my job in their own way.
I have asked my supervisor about what happened, and they have said that likely those colleagues just came with those ideas themselves. When I have asked, why did not my supervisor tell them to connect with me, as I have already did all this research, and we could have at least write this paper together, they have replied that they has forgotten about those works I have sent them. And did not suggest any way we could move one from there.
I think I made some mistakes myself: was too confident if not about technical skills, than amount of those required to get the paper accepted; I have not published that preprint even on arXiv, as first I have waited for the green light from my supervisor, and then I got distracted with my other job. Yet, I can only hope I will learn from this situation some thinks for myself from my own mistakes, but I still think my supervisor's behavior is rather unethical. My supervisor also works at one of the top 10 universities worldwide overall and in this field, and I certainly would not like my supervisor to teach students there this kind of behavior. So what can I do?
What do (or rather, don't) I have now:
there were emails with my drafts sent to my supervisor's email account; however, I have sent them from my university email address and that got wiped out after I have finished my PhD;
the paper published is not correct in it's correct shape: there are mistakes in the proofs - at least of the kind that stronger assumptions are needed to make the result hold, and then a rather big rewriting of the proof itself, at least in a couple of places, I did not really look into all places where those mistakes might have appeared, so maybe more, maybe not. It is a rather reputable journal, by the way, with one of the highest impact factors in my field.
In this paper the reuse a relevant result discovered by me, from a paper I have published together with my supervisor, without mentioning it (and essentially rederiving it anew).
What do I look for: it would be good to have this achievement attributed to me, but I am not sure I do formally deserve that (I've developed those results, correctly, but did not publish them). My main concern instead is that I unlike unethical behavior, especially in academia, so maybe some discussion with a commission on ethics would benefit my supervisor, or in the worst case awareness of the community (even though that sounds a bit nasty and dirty). I have checked out the answers in a related question, however I am not sure they apply readily to my situation.