A couple of weeks ago, the 800A breaker that supplies the feeders for lighting tripped for no apparent reason. Interestingly, this was during a meeting at which your boss discussed with the division VP your proposal for installing a moderately rigorous power monitoring system. However, since this was the only known instance of a nuisance trip, the VP wasn't swayed. Last week, the breaker tripped again for no reason. You had a tech run it through the recommended testing using the recommended breaker testing rig. It passed with flying colors.
How can you find out what's going on here?
If you don't have a portable power analyzer, get one. Or, hire a qualified testing firm to use theirs. This may further boost your case for getting approval for that power monitor, but the immediate concern is to begin capturing trip events such as transient power anomalies. You also want to see what's going on electrically at the breaker. For example, check power factor, harmonic distortion and waveform issues, and current and voltage on each phase. Since only this one breaker is tripping and the load (being lighting) is probably constant, the cause probably isn't a power anomaly. But you can't rule this out unless you are capturing power quality data.
You can buy a lot of cappuccinos for the cost of an 800A breaker, so don't jump to "repair by replacement." Have a qualified tech inspect the panel and perform a complete inspection, lubrication, and testing procedure on that breaker.