https://www.bleepingcomputer.com/news/security/thermanator-attack-steals-passwords-by-reading-thermal-residue-on-keyboards/

A person's fingers leave thermal residue on keyboard keys that a malicious observer could record and later determine the text a user has entered on the keyboard, according to a recently published research paper by three scientists from the University of California, Irvine (UCI).

"It’s a new attack that allows someone with a mid-range thermal camera to capture keys pressed on a normal keyboard, up to one minute after the victim enters them," says UCI Computer Science Professor Gene Tsudik, one of the three researchers who worked on the paper.