Even if you did lock it, the most likely people to steal something out of it are TSA employees.
http://sanfrancisco.cbslocal.com/2013/05/23/sfo-baggage-hand...
(SFO is notorious and most of the airlines keep lax enough records that they'll even deny that you checked a bag in the first place, much less that the contents were anything more valuable than old t-shirts)
In Europe you get the tracker code of any luggage that you have checked in.