I have never seen that happen and don't know how it could happen since the site id is determined either by the host name or the folder name on every request depending on whether using host name based multi sites or folder based. The sitemap that the menu binds to is based on the site id. If no matching site is found based on host name or folder name it would resolve to site 1 by default, not site 7, so that is very strange indeed.
Only thing I could guess is if a proxy server of some kind or an IIS module is involved in the request and changing request headers or something.
Best,
Joe