I suppose google yahoo robot crawl my site before but i check who's online and i found those visitor com from various IP. Some of them are from Web hosting company IP , theplanet, hostgator, globalhosting, others does they have robot? Adulu.com
Maybe someone post a link on a high traffic site as reference to a post or something on your site.. Might try checking the sites that contain link to yours. If it shows up yet? Boulder
For the sake of your sanity don't go trying to identify every bot that visits. I had a website doing that once and it was a bottomless pit trying to work out who came from where. Some of those will be search engine bots, some will be email harvesters, content scrapers, private search tools, experimental search tools and the list goes on. Others may just be picking up the RSS feed for your forum.