Detecting Hardware keyloggers... elegant solutions?

My company recently discovered a hardware keylogger installed during an routine maintenance procedure.

We tried catching the culprit but unfortunately it doesn't seem like we are going to be able to, so the culprit is still at large.

We're looking into ways to prevent this problem in the future. I did some research and it seems like some endpoints security software advertises the ability to detect keyloggers. Does anybody have any insight into how reliable this?

Another solution I have in mind is perhaps we could monitor for if the keyboard ever gets disconnected. However, would it be possible to continue monitoring the keyboards when the computer is turned off?

Please let me know of any thoughts you guys might have in dealing with this problem. Outside of gluing the keyboard into the port or physically securing the premise (unfortunately impossible), I'm quite stumped!