Idea of what's leaking?
Maybe your friends are smart alecs and poured trans fluid on the ground under your car while you were away? All kidding aside, sometimes when a transmission sits for a long time, the fluid from the torque converter drains back to the pan, raising the fluid levels high enough that if it was slightly overfilled, could burp out excess, or more likely, high enough to reach one or more bad seals.
I started it for a few minutes and there was double the fluid on the ground..
Well now you need to locate the leak, could be a broken or cracked cooler line, a front pump seal, as those two are the most common transmission leaks that can loose fluid quickly. Of the two the cooler lines will be the eaiser repair. The front pump requires transmission removal.
Yes- your transmission is leaking-
Looking for a Used Sierra 1500 in your area?
CarGurus has 65,798 nationwide Sierra 1500 listings starting at $1,750.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts
Related Models For Sale
Used Cars For Sale