Just a wild guess, as most of the other ideas have proven no good.
Take off the grill to the air register to the bath room. My wild guess is the past owner used that duct as a place to keep his stash safe. Bathrooms are the perfect spot for someone to hide something as he will be left alone to get at it.
It won’t hurt to look down the duct anyway with a flash light and tell us if it looks clean.
The only comment that has me thinking is how does Neal know “the eathy smell of mold might be cunfused for grass smell” Hmmmm