There's a box back there, a stranger tells me from his car

This morning, while walking Toebzilla along a main interior road, a man in his twenties drove his white car out of a rotary in my direction in the far lane.  I only took a glance at this vehicle, then diverted my attention back to the act of walking my dog.

Hmmm, I get a weird feeling, so I look back at the now slowing white car.  The man has his window down and seems as though he's trying to get my attention.  When we make eye contact, he announces to me, "There's a box back there", as he points off into the distance behind his car.

Without missing a beat, I yell back, "Alright!" in a tone that suggests ambivalent interest.  The man appears to be satisfied that he dutifully notified someone of this extremely important information.  He drives on his way.

He has indeed accomplished his mission, as I'm mildly curious regarding the presence of a box.  Is there something in the road that poses a risk to other drivers?  Is it a dropped package on the side of the road that was supposed to be delivered to a neighbor's home?  Why would this man be so concerned about a box "back there"?  

Remembering which way he had entered into the rotary, I turn around to walk in that direction to look for this mystery box.  Toebzilla resisted at first.  He knows our regular route.  After a brief protest, he relents to walk by my side.

I see nothing out-of-the-ordinary.  There's no box on this or that side of the road, nor within the lanes. Then, as I round the corner to walk down another street in the direction of my home, I see it.  I see the only thing out of place.  It's not a box.

It's a lonely shopping cart abandoned on the side of the road, across the street.  I look around with more intent to see if there was anything else even remotely nearby.  Nothing. 

Now, a thought came into my head that may be a leap in logic, but not necessarily an illogical leap.  Was the object about which this man was so earnestly trying to warn me not a box at all?  Rather, was it this shopping cart?  If so, why would this guy care so much about something so innocuous?  Did he hit it with his vehicle?  Did he already move it out of the way?  Did he have a brainfart, referring to this cart as a box?  

I will never know.  After realizing there really was nothing of interest and no good-citizen task being demanded of me, I lead Toebzilla back home.  One thing is certain.  I am now in possession of a completely useless bit of information.  This is too heavy of a burden to bear on my own.  I must now share this goofy experience with the world.