Is it okay to work on colleagues' ideas after they leave academia? I've just discovered that a former colleague is now working on some research ideas that I developed and laid the groundwork for now that I have left academia following failure to obtain funding. I'm happy that the work is being done, but feel that it would have been courteous to let me know that they were doing so. Are either of us in the wrong here?

History:
I did my PhD and a 3-year postdoc in a UK lab where we were expected to act very independently. As such I developed my own line of research during my PhD but didn't have time to publish the results until after I'd completed. I wanted to keep working on that line of work, so wrote a grant proposal with my supervisor, who acted as PI (I was named postdoc) which we were awarded.

During the postdoc, for complicated reasons, I developed another strand of independent research, with some guidance from a research fellow in the lab.

This strand then formed the basis of a large fellowship proposal that I was nearly awarded, but ultimately was unsuccessful. The fellow in the lab (who had a proleptic lectureship) acted as the academic host for the proposal.

Following a number of unsuccessful fellowship proposals, I undertook another postdoc for a year, then decided to take a different job and am now a software engineer at another UK university.

Situation: I've recently seen on twitter that the fellow who I had been working with (now lecturer) is now working on the project that I had proposed for my unsuccessful fellowship bid. We've had little contact in the intervening time and they've made no attempt to discuss this with me.

I know that it's kind of clear that I'm following a different path, and I'm happy to be doing so. But I'm a little upset that they wouldn't even send me an email to say "Hey, remember the stuff you came up with, well we got some money to do it etc.."

Am I wrong to be upset by this?

It's a small field, and my experience of my 7 or so years working in the area was that we all tried to be collegiate and courteous and not step on each others' toes. So this seems a little out of sorts.

I don't think there's anything to be done about this, but it has got under my skin for someone I trusted to have no qualms about using ideas I worked to develop whilst I had to leave the field to find work.
Is it okay to work on colleagues' ideas after they leave academia?

I'm going to be blunt: yes, absolutely yes. What is the other option? That the progress of humanity should be stifled because you didn't get a job?
Ideas do not belong to anyone. The way I read it, you did not even come up with the idea independently, but &quot;with some guidance from a research fellow in the lab&quot;. Look at it from the fellow perspective: they helped someone develop a research proposal, the person didn't get a job and the grant wasn't awarded, the person left academia, and (correct me if I'm wrong) made no effort to keep in touch with the fellow and/or work on the topic again. What is the fellow supposed to do now? Trash the idea, waste all the time spent helping the other person?
This happens all the time of academia. Supervisor helps a student/postdoc develop a research proposal, the proposal fails, the student/postdoc leaves academia. Now what? Is the idea not good anymore? There's no reason not to continue working on it.
Now let's answer your apparent actual question:

I know that it's kind of clear that I'm following a different path, and I'm happy to be doing so. But I'm a little upset that they wouldn't even send me an email to say &quot;Hey, remember the stuff you came up with, well we got some money to do it etc..&quot;
Am I wrong to be upset by this?

I'm not going to tell you that you're wrong to feel something. But if the question is &quot;did the fellow act reasonably&quot;, I would lean on yes. It depends what your relationship was, but based on what you've told us, the idea has been developed a long time ago (several unsuccessful fellowship proposals + a year of postdoc + since whenever you took the software engineering job, as far as I can tell), and it's not clear you kept in touch with the fellow. What is clear though is that you're not working on the idea anymore. At some point it becomes fair game.

I don't think there's anything to be done about this, but it has got under my skin for someone I trusted to have no qualms about using ideas I worked to develop whilst I had to leave the field to find work.

Academia and life in general is not just. Yes, it's unjust that you were unsuccessful and had to quit when you proposed this idea, but the other person is apparently successfully working on it. But this other person cannot change what happened or give you the job. I don't know you or your situation, but it sounds more like you're upset about having had to leave academia more than at your colleague.