Need Help! EVAP Lines

I was working on replacing the fuel tank pressure sensor on my 06 Chevy Avalanche 1500 2WD Southern Comfort Edition. As I was under the truck I was looking at the fuel filler neck and saw one small hose that had a small plastic cap on it just hanging from the filler neck. I can not figure out where it would attach. Can someone please look at the picture and it will be the smallest hose pictured in the square box with the plastic cap on it. Where it should connect.



Thanks so much!

It's the breather, It should have a small mushroom type cap on it. It allows air to escape the tank when filling so that the neck does not hydrolock and overflow. While you're under there check that it is clear, the old mushroom cap style has a tendency to get blocked up and cause fueling issues, especially with the auto/stay open pumps. Then new replacement assemblies have a "box" type cap with venting in it that is less prone to blockage.