I found this under the 2016 Wrangler today after washing it. I think but am not sure it was in the rear area. I believe it in fact it came from the Jeep, one of you will know what it is.
I remember plugs for the transmission dipstick holes looking like that on rebuilt transmissions. Normally removed and discarded. Factory transmissions had a plug that falls into the pan when the dipstick is installed. Let us know what you find out.