According to Alaska Airlines the smoke "appeared to be (from) the battery compartment of the device".
The Point-of-Sale maker, Toronto-based GuestLogix, has
recalled all devices of the same make and model.
Interestingly GuestLogix offers a
NiMH battery pack for airlines wishing to replace their Lithium Ion based batteries.
Fire from a thermal printer (other than battery/shortcut issue) is virtually unheard of.