Chevy K1500 oil leak

I have an oil leak from a mystery tube! It comes up from under the engine, and ends right next to the vapor canister. It's not connected to the canister, but in a clip at the side. The end of the tube has some sort of cap, and the whole tube is oily - it's making a small puddle on the garage floor. Can anyone tell me what this tube is, and why it might be leaking oil?

Thanks!