Suppose there is a building that was built in 1970, last rented out in 1975 and then bought by a company that has used it as their own offices until now. The last transaction was in 1975, what's the value if they apply for a mortgage today? Surely they have some formula to use for this based on e.g. other buildings in the area.

Moreover, "failure to find a tenant" is also a type of transaction. It's the landlord acting as the high bidder for the space, essentially the involuntary edition of imputed rent, and implies something negative about the financial prospects of the building when it continues for a significant period of time or large percentage of units. Ignoring that it is either incompetence or some kind of perverse incentive.