Given your update in post #30, I can understand your reaction, but I agree with the majority who say you overreacted. It just doesn't seem that hinky or out of the ordinary or out of the clear blue sky for the ice cream truck vendor to try to drum up sales from his truck by calling out the window.
And since you weren't standing right there where you could see your daughter, how can you know she didn't make some kind of gesture or something that was interpreted by the guy as a possible wave? Not saying she disobeyed you by purposely waving if you told her not to, but even the mere act of standing up could be interpreted as interest. And there are probably lots of shy children who need encouragement so asking if they want ice cream just doesn't seem to be at all sinister to me.
Had the answer been yes, the next step would have naturally been for her to seek out her parent or whatever adult and get the money.
As a mother, I know the mother-bear instinct can sometimes manifest itself unnecessarily and whien it comes to protecting my kids, I'd rather be unnecessarily rude than miss a signal and put them in danger, so I think the whole incident is forgivable. But since you asked, yeah, you oeverreacted.