I finally got the correct email address of the listing broker, here is the answer:
"The condo has to be rented because it is restricted as an investor-owned unit. In other words, the owner cannot live in the unit. This is permanent, as it is one of several in the complex that must be kept by owners as rentals."
The email continued and said that it was possible that if a family member owned the unit, it could be rented to another family member, etc.... more
One possibility is the owner rolled over a lower tax base from a primary residence they owned before this one under marin provisions for over 55 age homeowners. You could call tax assessors w AP # and ask them