Where luggage goes as a precaution (2,4)
I believe the answer is:
in case
I believe this is a double definition.
'where luggage goes' is the first definition.
(in a suitcase)
'as a precaution' is the second definition.
(just in case)
(Other definitions for in case that I've seen before include "Where traveller may keep clothes" , "For fear that" , "for emergency" , "Packed" , "As a precaution" .)