REMOVED is just a synonym for LOSS.  I see no advantage to using it, it demonstrates no cause.  Using null instead of an explicit cause is not ideal either.  It'd be better to use LOSS or REMOVED as a null object pattern type of approach if necessary.

EXPIRED indicates the time ran out.  TREATED could indicate some form of counter acting agent was applied.  I'm not sure why this is hard for me explain.  A cause is something that explains why the event is occurring.  It is not a generalized state attribute.