An OS agnostic solution would be a keyboard hardware keylogger [1]. I have no idea who sells a modern version but I'm sure they must still exist. I used them extensively in the past to see if anyone was screwing with my computer. Some of them can transmit over bluetooth or wifi to download stored keystrokes.

Example [2] never tried this one

[1] - https://en.wikipedia.org/wiki/Hardware_keylogger

[2] - https://www.keydemon.com/en/keydemon/66-usb-hardware-keylogg...