$20 bills on the sidewalk

A true story:

Three economics graduate students make their way to the car park.
A rectangular piece of paper flutters on the pathway ahead.

One student walks straight past. He did not notice the paper, since he was not alert. He was not engaged in any optimising calculation, and missed a moment of costless discovery. (Ironically, this was after a class on Austrian economics.)

The second one notices the paper. But he thinks that if was a $20 bill, someone else would have picked it up. Without investigating further, he proceeds. (He must be from Chicago.)

The third notices it, and invests resources by stopping to have a look. Upon close inspection, he realises that it's a piece litter.