What is the nipple on top of & on rear passenger side of air intake manifold?

I've had codes P0171 & P0174 for about a month. Smoked the whole system and found a leak coming from a small (1/8") diameter nipple shielded on both sides by rectangular plastic tabs.  I put a plug cap on it and all is well.
I've looked in manuals and on-line and cannot find what this is and if a small hose or something else was attached to it  Don't remember knocking around in there to cause something to fall off.  I have a 2008 LTZ
Thanks for your help!

Any chance you can post a picture?

Sorry, meant to post pics.  Here it is.

According to the service manual nothing attaches there. I'll take a look at mine tomorrow and see if it's capped or not.

Checked out my truck. There's supposed to be a plug there.

Nitrous port, perhaps?

Nitrous, I wish
Thanks for the cap answer.  I cannot imagine how the cap got dislodged or broken.  A caliper bleeder valve rubber cap fit perfectly.

I wonder what is the purpose for a capped port on top of the intake manifold?

As always, thanks guys!