Standard 15A circuit breaker trips randomly under no load
Even though you call them CBs, I'm trusting that you know the difference between a plain breaker and a GFCI/AFCI.  If your breaker has a TEST button, this advice is no good. 

The usual reason for this, particularly in a lighting circuit, is a ground hitting a hot wire/screw, typically on a switch - switches can shift when you operate them.  It's either hitting the bare ground wire in the box, or it's hitting the side of a metal box. *

So if you remember a flash or sudden failure when you operated a switch, that's the first place to look.  

One thing we often see is people using backstab connections. Those are a lazy shortcut used by builders in a hurry, but they provide generally poor connections.  Those cause opens not shorts, so they aren't directly responsible for this. But it using backstabs means they didn't use the screws, and often the screws are left in the full, all-the-way-out** position.  These are much more likely to snag a ground wire or hit the side of the box. 

Once the screws are down (preferably with the wires under them and tightened to torque), you can avert further trouble by wrapping electrical tape around the perimeter of the switch (or receptacle) so the screws are covered up.  I do that to make them safer to handle. 

Another way hot wires trip is when they are wearing their "Daisy Dukes", that is, when the wire is stripped too much for the backstab, and leaving exposed conductor for a ground wire to find. That is bad workmanship, but then, so are backstabs generally :) 

* Metal boxes are very much your friend.  They provide a better safety shield for the wiring, they conduct heat away from hotspots so they stay too cool to start a fire, and (annoying though this may be right now) they assure trips in cases like this instead of leaving a problem to smolder. We've had people tear them out and go plastic in cases like this, but that's just hiding the symptom.

** screws have a detent/stop to keep them captive.  It damages the threads to unscrew them any further.  For screw-to-clamp types, it will make the anvil fall off, and the receptacle is scrap at that point!