Doug - this behaviour is by design. It's based on the way the rewriting works. When you have a page with a third party module on it, the rewriter inspects the url and finds a match with a known tab path. Anything else after the part of the url that matches the tab path is considered to be a query string.
So, if you have a page called /path1 and you request /path1/nopage.aspx, then you'll get the /path1 page returned as normal. The /nopage part of the path will be treated as a querystring.
At the moment, there's nothing much you can do about it, except for create a page matching the path you want to 404, then deleting that page. This would then match and produce a 404 under the rule for deleted tab handling.
Mostly it's not important unless someone links to a non-existent page. I am looking at ways to handle this better for future releases, but the problem is a tricky one. One one hand it's necessary to maintain flexibility of the Url scheme so that new additions to the site will work OK, on the other, some people want to lock down the urls very tightly.
My suggestion is that if you have a particular page which is being linked, then create/delete a page for that url, (or set up a redirect to handle the url), otherwise, it's not likely to be a big issue.