Mystery drain plug

Hello-
New here!

We have an inground Gunite pool with attached hot tub, that we used a company last year to open/close. They left all of the drain plugs, etc in the skimmer baskets as they should have. However I seem to have one extra plug! Everything matches up fine, with the exception of one drain plug that I can't figure out where it could possibly go.

We have 2 on the pump, 2 on the heater, the site glass and an additional plug on the multi valve port. And one big one at the bottom of the filter.

Our filter is a Pentair, Pump and heater are Hayward. Is there a mystery "plug" that maybe I'm not seeing that any of you may know of?! Please help, We want to open on our own this year, and are comfortable doing so...but not until we can figure out this mystery!

Mystery solved...there is a drain on the back of my chlorinator that I can't actually see but I can "feel"! Thanks anyhow!

Thanks for following up! Welcome.

. . . membership upgraded -- love the name of your town!

(I like 'real' names, rather than developer 'boutique' names, like 'ye olde quaint towne' or "Oakwood Estates"! One of my favorite place names from the area in N. Georgia where I grew up is "Booger Hollow". Few people realize that that name refers to Cherokee "booger" masks, rather than nose droppings ;) )

LOL- thanks!!