They had handguns in the display case overnight?
This is my first thought...
My FFL put all his handguns in a very large safe every night.
I thought part of getting your FFL was to ensure you had a secure place to store them.
That being said no one deserves to be robbed. Leaving the guns out - Stupid maneuver or not on the owners part ... thieves make my skin crawl. They never want to put forth effort to get ahead in life and have entitlement issues that nauseate me.