I found it! The SPIF (Spacecraft Processing and Integration Facility) was used for the Shuttle program, and renamed to SPIF after the Challenger incident. Used to be the SMAB (Solid Motor Assembly Building).
The IUS (Intertial Upper Stage) were assembled there for Shuttle payloads.
The location is shown on this map:
The internal layout looks like:
The outside looks like this:
There are several bays, with room for 2 payloads to be processed, a third in the transfer aisle, and airlocks to get the payloads in and out, without contamintating anything.
On the last image, bottom left there are three doors side by side. Two are access ways to two bays, and the third is the airlock for incoming payloads.
SpaceX is leasing space in the SPIF, apparently as they need it (Used for Asiasat 8 and 6 launches, may be used for Dragons as well to make space in LC40 hangar and allow for two or more payload processing flows at once.
Here is a picture of a DSP being mounted on a IUS for a Shuttle launch:
Here is a picture of the outside of the building.