I have seen those folks for at least six months or more. They are not DPD and they don't have fancy EDACS radios, they appear to be simple analog. I've logged several low-power frequencies that could be related to 16th Street Mall, most notably:
*461.6875, 205 DPL
*462.1750, 051 DPL (Independence Plaza?)
*463.5875, 371 DPL (possibly LODO-area vendor operations or security)
I suspect it may be difficult to confirm this frequency without hanging around close enough to hear the radios directly. In that area there are TONS of simplex or low-power half-duplex UHF frequencies; I've logged dozens of them. If you sit nearby with headphones and put your scanner in search mode from 450-469.9875, you should be able to find the freq. they are using if you wait long enough.