I just drove right by the entrance of the Silver Lake Costco in Everett, Washington, looking for any such sign, using the above poster's location notes.
There are no such signs. I have not seen any such signs on any Costco here in Washington state. This again leads me to wonder if this is something unique to Arizona or some other states due to something in their state laws.
The only signs related to firearms carry prohibitions that I have ever seen in Washington are on bars and taverns, courthouses, schools, Federal or State government buildings, some healthcare facilities and military installations. I just don't see them on other private businesses, unlike I did in Dallas, where I just returned from a seminar.
Snohomish County, Washington USA