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.