Are there any sexually transmitted infections that a dude can have without knowing?

Absolutely: Most infections have a period of time where you don't have symptoms. For example, with hiv, most patients don't have any symptoms right away. Weeks later, you may have some symptoms which resemble a generic viral illness. Bottom line is, you should get tested.

Many: Without going through an extensive discussion of each and every std there is, many can present in an asymptomatic fashion. The most notable are syphilis and HIV disease, but there are others. If you have been impetuous enough to have become involved in unprotected sexual activity, go see a doctor and get tested. Good luck.