"echar una cana al aire"

According to the thread I've found, "echar una cana al aire" doesn't necessarily mean that you are unfaithful. From that point of view, letting your hair down or having a fling have the same meaning. To my mind, "echar una cana al aire" has the connotation of being unfaithful, in which case, it means "having an affair". Have a look at the three threads on the topic at the end of the page, in the forums. I'd personally go for the third one:
http://www.wordreference.com/es/en/translation.asp?spen=echar%20una%20cana%20al%20aire