Were they all nicely placed like that?  IF not, you may have messed up a potential crime scene.  Legally, you can't connect the litter to the license plate.

Wow.

I placed all the items together for a photo at the end of the walk.

But now that you bring up the possibility, here's a scenario that could connect all the scattered items:

An angler is motoring up north, boat trailer in tow, window down, munching on a Deli Express sandwich and pleased at the great deal he just got on epsom salts at Walgreens. And there's that great deal on copy paper at Office Max to hit on the way home.

Wait! There's something in the road! Crunch! He swerves, but still crushes a child-size guitar to oblivion. As he does, the trailer's license plate is jarred off and his tasty sandwich slips from his hand and out the window.

He pulls to the side of the road, and as he opens the door his Walgreens receipt and Office Max note fly off in the wind.

He goes on with his trip, doesn't catch anything AND forgets to go to Office Max on the way home.

How's that?

I like your proposed scenario akjuneau.
If that is all you found on your walk then you are lucky.
I stopped this morning on my way to work to look at the sparkly lake. 
I was amazed at the amount of refuse located at the rest stop. 
Although there was a garbage can in the area, someone carelessly left a bunch of garbage in a pile by the lake.
Do the litter bugs of the world feel guilty (or does their laziness cloud their judgement)?

Re: the epsom salts, I'm thinking perhaps a horse owner/farm owner may have purchased them since Rice Lake has more barns than any other Duluth area. When a horse gets an abcessed hoof, you soak it in an epsom salt solution.