[marginalia_nu]

Could be, but usually bots that post spam include some form of link, not just keywords. The link is the entire point of posting spam.

There also seems to be a few distinct bots doing this, with distinct user agents (but bots with the same UA spam-search on the same topic out of epharma, casino, warez).

I also have another, identical search box in another part of the site and it has never seen any sort of spam bots.

[panda88888]

I wonder if these are attempts to drive up the “interest” metric, with the hypothesis that the more search for a specific string, the result that is exact match gets higher ranking?

[marginalia_nu]

Could definitely be something along those lines. Discovering all this black hat SEO stuff and what they are doing to manipulate search engines has been one of the great joys of this project.

Nobody seems to write much about it, maybe because they don't want to give people ideas, but it's an endlessly fascinating subject.