Is it okay to work on colleagues' ideas after they leave academia?
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.