I don't think they test for Herpes when doing your standard STD test. I think in order to test for herpes they need fluid from a blister. I don't know, all I know is I went to a reputable doctor, he took one look at me and said, that's herpes. I was actually relieved because it's not that bad to treat, although the initial outbreak was kind of a pain in the ass. I also felt better because I knew where it came from. My girlfriend and I don't have STDs, but she did have oral HSV, and must have been shedding the virus without showing signs because now I have it. Anyway, I've been reading a lot about it lately, if you're interested you should read this guys lecture, he seems to know what he's talking about.
http://www.herpes.org.uk/art_kinghorn.html