Googlebot issues

When I checked my latest visitors logs in cpanel, I see the following:

/?p=28&preview=true (This is coming from where?)
Http Code: 200 Date: Nov 10 00:52:53
Http Version: HTTP/1.1 Size in Bytes: 2347
Referer: – Agent: Mediapartners-Google/2.1

See the first line which has /?p=28&preview=true (which actually works as well) and it also was served with Http code of 200. Does it mean that this URL will show up in Google? I had seen this problem before with one other website of mine and Google had many such URLs ignoring some url_rewriting I had done.
I think, WP should return 404 for /?p=anything… (not sure if that wld be good)
My website for reference is http://www.bloghash.com

Thank you!