Well, I am the SAR team. ;-)

I would investigate. If i'm within hearing distance of the whistle, I believe I would be able to get closer and closer to the location. I would definately take my time though, stay put and concentrate on the whistle sound, I wouldn't run around like crazy trying to find the person without paying attention to the whistle sound. This way, I think I lessen my chances of getting lost.

I would administer first-aid if required and give the person something to eat and drink. I will give them all the gear I have that might be helpful : AMK heatsheet, firestarting material, etc.

Then, see if the person can walk, if so I would try to bring them back to safety myself. If not, I would go back to the trail (assuming there might be one) and place a signal of some sort (flagging tape with message written on a piece of paper, along with an arrow made of branches). If I meet someone, i'll ask them to go for help while I care for the person. If not, well I will go get help myself.

Along the way, if cellular service is available, I will organise the evac by calling 9-1-1.

:-)