I know I am a new member, but I have lurked for a while. I also frequent a few other forums too. You may want to check into this campaign:
# 15183: Service Update for Inventory and Customer Vehicles - Driver's Side Front Roof Ditch Molding Water Leak - (Mar 18, 2015)
Subject: 15183 - Service Update for Inventory and Customer Vehicles - Driver's Side Front Roof Ditch Molding Water Leak - Expires with Base Warranty
My 2015 Yukon XL sprung a leak on the driver side in this area. Took it to dealer and they suspected the sunroof leaked or something. Never saw a leak again. The campaign has you remove the plastic strip molding on the driver side roof in front of the rail. It is in the little track there. If you remove that, the campaign seals up the area with some RTV and then puts the molding back in there. Apparently the tack weld between the roof and side panels isn't quite water tight.
This might not apply to yours, just figured I would throw it out there. Good luck