This is the bonus cache for the Historic Swansboro Adventure Lab.
https://adventurelab.page.link/EhjA
Geocaching.com's Adventure Lab caches can only be found with the use of a smartphone or tablet using geocaching.com's free Adventure Lab app. After completing all 5 Adventure Lab caches, the app gives you the coordinates for this mystery bonus cache.
It should be a pretty easy one to find. Happy hunting.