If you are going to run code on the computer you are logging from you dont need a teensy, you need a USB flashdrive to store keypresses on.
I just ordered my own Teensy and have thought about this myself.
As far as i know, the PS/2 or (very) old RS232 keyboards utilized a standard serial line to transmit keystrokes.
So with that in mind, you could connect a PS/2 to one of the serial inputs of the teensy and program it to pass the data from the PS/2 to the computer via USB (and in addition, store it on the teensy using the SD adaptor)
Then you could either make it look for a certain combination of keys that would dump the entire log or just store it on the teensy, for when you are around to retrieve it.
I guess this wont work for laptops and computers with a USB keyboard. But it might be the safest way to do this. No antivirus software in the world would be triggered, and the only thing giving it away is the USB HID device connected to the computer.
You would have to have access to the computer for extended periods of time, twice.
