I assure you there are time when that happens to me, and there is absolutely nothing going on or around me that would cause that arousal. And I'm not even thinking of such things. Yes, it happened a lot more when I was younger and suffering from "raging hormones." And yes, it can be embarassing at times. If I'm lucky, I can pull my shirt out so it covers the "evidence." I'd look a bit silly holding my hat down there, and walking around.