investigate BadUSB attack
There is lots of talk about it being impossible to scan for malware that uses this exploit, or remove it. But that seems dubious. If you can flash a microcontroller, you can re-flash a microcontroller. And you can usually read the firmware from the microcontroller as well. Sounds like the worst attack a bad USB can do it output stuff as if it was a keyboard and/or mouse. But that would clearly show up in the UI of the machine its plugged into.