So I was clearing Coastal Cottage and just north west from it is a Hatch. With the option to detonate which just opens it. But you can't go in. It just stay a jar until you close it. Any ideas what it does?

It's a random encounter. I've seen it twice. Once broken, and another it worked. There was a scavenger outside and he became hostile as I approached. The hatch explodes and I survived only thanks to my Dense modification to my armor, and it's a pre-war bunker full of loot.