I love writing Flash Fiction, using the variety of the challenges as a way to practice my writing while expanding my horizons. In addition, if I see some theme or a group of characters reappearing, that's a signal that there may be a novel in there too. I publish these little stories on my blog, along with the topic involved, and have received helpful feedback on many of them. In addition, hopefully I have encouraged others to try them too.
When I do use some of these as a base for a novel, I re-write and expand the stories, along with the addition of much new work.
However, before I start sending my novel to agents, should I delete all the original stories online? (1)
Should I leave them there and advise the agent about them?(2 and 2a)
How you come up with your ideas for your novel is not something I need to know about and often it's not something I WANT to know about. Kind of like making sausage, or Donald Trump's hair. The less I know about how it's made, the happier I am.
When you query me, I'm interested in whether you've written a book I want to read. That's pretty much the extent of my interest.
Later in the process, when you're doing promotion for your book and interviewers ask where you get your ideas, you can talk about the value of flash fiction.