That brings up another good question (in addition to my earlier question about how much money is typically involved): Are these super early-stage acquisitions usually a result of the buyer spotting a diamond in the rough and reaching out with an offer? Or are they usually a matter of the early-stage startup just knowing the right people?

I suspect the latter, but it's purely a guess. This is one area of Startup Land about which very little has been written.

I think spotting, for example lookout outlook plugin acquired by MS, which seems clearly to me MS needed such solution and lookout already solved it nicely. So why re-invent the wheel and spend more money when you got a safe choice.

I think Lookout was a talent acquisition. The current implementation of Outlook search is much worse than Lookout was.

They didn't integrate into the Outlook though, they did integrate into Windows Desktop Search

Outlook search is powered by Windows Desktop Search I think.