Since it wasn't actually an NFA item, I guess the federal pound-me-in-the-ass penalties don't apply...
Depends... the ATF has some pretty vague determinations/rulings regarding silencers...
http://www.atf.treas.gov/firearms/nfa/041708silencer_faqs.htm
If the pilfered item was an actual registered tube with all the markings, then the ATF can consider it a silencer (even though the internals are missing).
Probably along the same idea how they ruled that a full auto sear is a machinegun.
Years ago it was common for different vendors at gun shows to sell differnt silencer parts. A buyer could go to one table and buy the tube, then another table to buy the baffles and it was legal... that was until the ATF wised up to it.