I have a '14 and I could have sworn I heard the dreaded sloushing sounds but actually thought it was my imagination because the truck is so new. Now I really need to check this out based on this thread. Geez. If this is in fact the case, I will be livid.
If your radio is on or someone is talking with you, you might not notice it. Make everything quiet, get in the driveway or other smooth surface. Drive forward 10 feet or so and step on the brake. The speed doesn't have to be fast -- just enough to force the water back on mild acceleration and then back the other way upon braking. The sound is pretty much unmistakeable.
My car sounds like i'm in a bath tub. It had about 2 gallons of rain water underneath passanger side. Pulled the plug and it all came out. However the driver side has a more difficult plug so I will take it to dealer.
FXAFELIX called it!. The service guy at the dealership diagnosed the water collection as the result of a failed sunroof drainage system and planned to have to remove the emergency brake to access the correct panel as well as the seat and carpet. When I took my GC to get the work done, I carried a copy of FXAFELIX's pics and some of this thread describing what other owners had experienced. When I picked up my GC, the service guy said that the problem was, in fact, the ac condensate drain. Problem solved! I hope!
2014 GC Summit, built in March. Heard the sloshing sound for the first time today. We've been having a crazy amount of rain over the past week. Going to pull the plugs as soon as it stops raining and will have the a/c and sunroof drains checked out as well.