It's a true ungrounded 3 wire Delta.
Since the facilities are critical we can't shut them down unless we do a major upgrade so most of the existing Services are the original ones from the 1930s...or earlier.
One of them still has the oil insulated vertical drawout cast iron MV circuit breakers.
Major upgrades are designed with a conventional 3Ph4W system.
the reason i ask is you said this about the indicator lights
There are 3 lights wired from phase to a ground rod and all glow continuously at the same brightness.
If one of the phases has a ground fault that light will go out and the other 2 get much brighter.
i don't think the light would light up if it was completely ungrounded,
this light set-up is what is used on high-impedance grounded systems, a first ground fault will only be low current like 5 amps(i don't remember for sure). the lights do the same thing you are describing.