My former agent and I recently parted company on good terms. Without going into details, we inherited each other when his partner left the business. and weren't really a good fit.
I am currently completing a non-fiction proposal, which was not completed at the time when we said "bonne chance and farewell" and which he did not send out. Do I mention previous representation in the query or bring it up later in the process?
You do not need to mention a former agent for a new project UNLESS you've been published before.
If you have been published, you'll need to mention the earlier works, the publisher and that the agent who handled the deal for you left the business.
If you haven't been published, you don't need to mention any previous agent relationships since this project did not go out on submission.
Of course, IF the project had been shopped, you need to say so, and by whom. This is when you need the submission list and editor's replies. On some occasions, I can see holes in the submission strategy and figure out a better plan. (That's NOT often at all, sorry)
Make sure you update your website if you have the old agent's name on it. That's one of the easiest ways for me to discover that someone has been represented/published before if they don't mention it in the query. And yes, I DO look. If I'm interested in reading your manuscript (or in this case proposal) I do some sniffing around ahead of time to see if there are any bumps in the road.