I'm seeing that in Awstats for all my sites too, and I didn't find anything resembling that UA in the raw logs.
So I went to the logs for a site that gets very few visitors and selected all the visits for 6 Feb (the date of the "last visit" for bot[+:,\.\;\/\\-] as reported by Awstats). I then removed the requests for pages and supporting files, and was left with 15 requests for robots.txt by Googlebot, Bingbot, Baidu, SBider, Yandexbot, and Slurp.
Bing accounted for 9 of them, Slurp 2 and the rest 1 each. And all of them have been back since 6 Feb.
None of that is out of the ordinary, so I think that Awstats has somehow got itself in a tangle. It looks like a snippet of Regex code.