Turns out one of our UPS did complain about "lack of bypass" at 19:02 on Friday and again around 08:33 on Saturday, but it did not declare "on battery" as that generates an email at the time.
Odd thing is we have 3 UPS, each off a different phase, but one is much older so reporting different, and the other did not complain. We more or less have a direct path to the local substation (110m from our area to the 100A feed switched-fuse off the 800A busbar chamber, in turn fed via about 20m of singles from the 500kVA transformer) so it is odd, but maybe it was seeing major disturbances just on the one phase.
Or the two UPS somehow have different internal thresholds, etc. They should be the same, but were bought about 6 months apart.