My 25 year old 3 level townhouse's slider in the basement level is leaking from the top of the frame on the fixed glass side. There is no evidence of moisture on the interior walls on the two levels above the door, nor on the wall above it. The roof looks fine on the outside. There is a deck above the slider and the exterior is alumium siding. Any suggestions about a cause, fix, or what type of person to call for repair?

Siding repair people can diagnose this quick enough, I would say it would be on the drip edge right above the door itself(common area for water to collect)

Or perhaps the deck's connection to the house(again the siding repair person can see that as well),that connection has to be caulked to insure no water leakage.

A picture would greatly aid us in an online diagnosis.