"And that something peculiar had occurred in the room-sized vault in Holmes's building, a phenomenon when finally discovered by police three years later would defy scientific explanation.
Somehow, a footprint had become etched into the smooth enameled finish on the inside of the vault door at a point roughly two feet above the floor. The toes, the ball, and the heel were so clearly outlined as to leave no doubt that a woman had left the print. The degree of detail mystified the police, as did the print's resilience. They tried rubbing it off by hand, then with a cloth and soap and water, but it remained as clear as ever.
No one could explain it with any certainty. The best guess posited that Holmes had lured a woman into the vault; that the woman was shoeless at the time, perhaps nude; and that Holmes had then closed the airtight door to lock her inside. She had left the print in a last hopeless effort to force the door open. To explain the print's permanence, detectives theorized that Holmes, known to have an avid interest in chemistry, had first poured a sheen of acid onto the floor to hasten by chemical reaction the consumption of oxygen in the vault. The theory held that [redacted] had stepped in the acid, then placed her feet against the door, thus literally etching the print into the enamel.
But again, this revelation came much later. As of the start of 1893, the year of the fair, no one, including Holmes, had noticed the footprint on the door."
Source - Devil in the White City