On Friday, 27 September 2019 09:54:45 UTC+1, wrote:
Three phase DB. Turned up and tried to turn the lights on, to find that all the MCBs controlling circuits with lights (and some without), plus a submain that also had a lights circuit running from the secondary DB, we're off.

The only possibly explanation I can think of is that someone had turned them all off, because they were blindly trying to turn off the lights after working there and hadn't noticed the timer override master switch. Problem with that theory is that in order to turn the lights on in the first place they would have had to know about the override switch!

Is there any possible electrical fault which could trip half a dozen circuits and leave no signs, a maximum of 24 hours since the lights were circuits were last on. There was no evidence of water leakage.

If they have tripped, the toggles would be in the mid position. To reset/turn back on, they have to be turned to the fully "off" position and then "on".
So it should be obvious if they have tripped or been turned off.