I have been shopping at Costco since the first store was opened south of downtown Seattle back in October 1987. I can say that none of the Seattle-area stores that I frequent have any signs whatsoever about firearms or carry thereto. I wonder if this is an Arizona phenomenon due to some unique state laws. I happen to be in Dallas right now for a seminar, and many businesses here have signs at the doorway prohibiting firearms carry on the premises. For that matter, most bars and taverns back home have similar signs.
I can further say that my brother once did some custom glass work at the Issaquah home of a Costco senior vice president. It turns out that the senior vice president was on the US Navy pistol team, and my brother in fact sold him a tricked out 1911.
Snohomish County, Washington USA