when I was in college I used to work at a public park. one of my jobs was picking up litter alongside the highway. I used to find $1 bills, sometimes even a $5 or a $10 sometimes. of course I kept it.
but a whole brinks bag? I'd be afraid they were marked bils, or that the contents were otherwise traceable, and I'd turn it in.
I'd hope for a reward, but I'd expect at least a "thank you."