If you recently 301-ed other domains to your central site, this may be URLs of old pages which existed on one of the other domains in the past and which still have links pointing to them from other sites. People may occasionally follow these links.
You probably now return a 404 when these URLs are requested. You may think about finding a matching page on your current site and do a direct 301 from these strange URLs to your existing page. In that case you can preserve the link juice these links may be carrying and the user experience for the visitors will be better then when they are sent to a general 404 error page.