The Pinkerton Agency is probably the world's most famous detective agency. They had offices everywhere in the U.S., and were involved in all manner of security work as well as investigations.
This vintage porcelain sign was on the door at the former Erickson Jewelers shop in Andersonville, a neighborhood on the north side of Chicago. The shop's neon sign hung there for quite some time after the shop closed as well. The space has since been turned into a restaurant, and I don't know if this little sign is still there. I rather doubt it.