After a bit of field work, it appears they are probably "amplifying" the squaw mountain DTR site. However, there were certain areas of the mall that had no coverage from any P-25 system in 700/800 band; this includes virtually the entire food court area, which you would think they would want to cover with an in-building amplifier. Perhaps in certain areas they only amplified the "repeater input" signal. In any event, I've NEVER heard anyone complain about radio signals inside the mall, so I'm still thinking I could be missing something.
Maybe they're doing something funky outside of DTRS, such as tying the in-building antenna site to a conventional 700/800 channel pair and back-hauling the "other side" to the dispatch center with a cable or microwave link, instead of an exterior antenna. That would take a bit longer to find, because I would need to search for a intermittently active conventional channel, as opposed to a P-25 CC which is always active and is easily identified with my 396XT's "DAT" flag.