Mystery weapon gone?

I found a weapon in a box at the Supply Depot Lima that automatically equipped. But when I found it I didn't get like a notification at the top right of what it was or that o equipped it. It looked like a bazooka? It was a big weapon that looked like it shot explosives. I unequipped it and it's disappeared? I don't have it in my inventory or anything or maybe I'm missing it somewhere? Plz help