The Joker had to admit that he had indeed tampered with the crime scene beforehand and taken a strand of hair from the corpse. That was admittedly his fault, but he had investigative needs too. Evidence went to whoever got there first; it was understandable.
But the Joker had been extremely careful not to leave behind even a single footprint of his own. The shoes he wore had no tread whatsoever. He had avoided stepping in any mud, and he had deliberately kept pressure off the soles of his feet to prevent instruments from detecting the full shape of his footprints.
So the only footprint at the scene was that half-print in the mud beside the corpse.
The Joker clearly remembered that the corpse's blood had flowed into the dirt, turning the originally dry soil damp. The shoe had landed half on the tiles and half in the wet mud, leaving behind half a footprint.
Note that the half-footprint left in the mud had depth and contours. It could hardly be called a print; it was more like a three-dimensional model. Where the sole's tread protruded, the mud sank in, and vice versa.
As everyone knew, three-dimensional things always drew the eye more easily. They were far easier to spot than some inconspicuous pattern stamped onto a flat surface.
Yet the Joker hid in the shadow of the prison van and watched Batman wander around there for at least a full minute without noticing that footprint.
He was certain Batman had not seen it and simply ignored it. Batman had not seen it at all from beginning to end.
The Joker looked around from inside the prison van, and in the end, he reluctantly concluded that perhaps the footprint happened to be directly beneath a streetlamp. There was no side