Maybe your thieves set gold-plated toasters and Warhol prints in place of the items they steal. Ala Indianna Jones.
Or maybe that's the Bad Guys' calling card - the equivalent of a horsehead in the bed.
I think I may give Secondary Character a POV after all. I can make him an FBI (or something) agent. The only reason I want to use him is because I have a great name - "Quincy Archer" - doesn't that sound exactly like a hard-bitten PI?