ok so we get this question all the time. first off, which icon are we talking about?
second, there are just too many apps out that uses notification icons. so unless it is something like youtube or facebook (something popular) you will most likely on get guesses from us. these threads never truly get answered.
just ask the person that the device belongs to as many of these questions comes from people who do not trust their significant other. if this is not the case, then you can just pulled down the notification bar to see what it is. or you can go to your phone's settings and look under notifications to find out.