[johnsjmc]

I had something similar in a rental 4 unit I owned about 25 yrs ago. That building had separate unit electric heat. I got a call from an upstairs tenant that the water was off. turned out tenant below had gone on vacation and left a window open with heat turned off. The cold snap arrived and pipes froze between units.
Turn off the main ,warm the building up and when you turn it back on listen for too much or any flowing .
Then when the system is pressurized go around and look for leaks. I would expect a leak or two given the copper lines. You could get a small drip or a fitting could have blown off completely with a huge leak as a result.
In my case with the 4 plex. water came back on to the upper apartment while I was there. BUT I could hear water running somewhere.
I went back in to the lower unit just as the ceiling fell down weighted by a few hundred gallons.
Hope you only have a couple small leaks and can get at them easily . I had insurance so the big flood damage was covered.
Read your policy and see how often you are required to inspect a vacant building BEFORE you call them if you need to. My policy says no more than 7 days when vacant but my daughters house policy says every 72 hrs.