Update.

I successfully used B270898 for my weekly rental from local office August 1-8. My contract lists "Peregrine Meeting Systems", a company which, according to Wikipedia, went through Chapter 11 bankruptcy in 2002-2003, and was acquired by HP in 2005. (The association of Peregrine with this AWD is mentioned somewhere up-thread.)

So this is not a Costco AWD. Autoslash's comment that this AWD relates to HP tracks, although why Avis still has this AWD associated with Peregrine in their system, and not HP does not make sense to me. In any case, I felt no fear that I would be asked to present an ID since the company no longer exists, and also had a parallel reservation with another rental nearby rental company, so I would have just rented there if Avis had not honored the rate.