Terrifying on the same level as Ken Thompson's "On trusting trust".
In theory even a capacitor could be listening for a serial activation
code and then go short or open circuit to modulate a new signal onto a
wire.
It's a real problem, to put it mildly. I've done enough work with electronics that I know I'd have a hard time identifying such a device or something more up to date and I have at least a basic understanding of what it would take to do this. Someone that is unwary doesn't stand a chance.
Where the hell does that leave us?