Originally Posted by Bocolo
Why would the breaker smoke if there was nothing drawing power from it?
An internal defect in the electronics circuitry of the AFCI.
Failing by shorting = smoke.
Failing by opening = no function, no smoke, no nothing.
Regular breakers don't have circuitry across the input lines, only a 2W resistor in series with the hot line.
I'd guess a working AFCI would draw 10 mA or so all the time just to power the innards. You could confirm this with a DMM on your other AFCI. Start on the high amps scale and work down.
If the thing is not returnable you could autopsy it and look for a charred component. Depending on what smoked you may be able to replace that component for a buck or so. Maybe.
But then you should only use this repaired AFCI for testing and whatnot.
Building a test setup to weed out bad AFCIs that fail in this way without smoking them is a bit complex.