I think it looks like Snoopy wins the prize. :)

 

Yep, no plug. Hole is there so that you can see what you are doing when you use your "serpentine belt tool". I guess GM figures nothing is going to get in there to muck anything up (and I think they are correct), or they would put a bottom cover under the entire engine/transmission.

 

No plug on mine either... noticed this the other day and hadn't had a chance to come back and update this thread....... (Okay, I forgot! I'm getting forgetful in my old age, shoot me..... Please! )

Anyways. The OP's looks like there was something there because some tape was applied when undercoating was sprayed. I think someone already mentioned this above, but there's not supposed to be a plug there.