Thats an illustration of the difficulty you have when you taking an electrical problem to the average garage - they often have no idea what they are talking about electrically.
it is possible that with extra lights etc you were using a bigger load than the alternator could deliver, you flattened the battery enough so that the engine would still run but the starter wouldnt turn over the following morning. possible but unlikely if the difficulty occurrs more than once.
its also possible that you have an internittent high resistance contact in the starter circuit - something that causes a high voltage drop. again unlikely on a newish bike unl;ess its been overwintered outside and has corroded with the salt etc.
but much more likely is that the batteries have simply discharged overnight. this can occurr two ways - the battery can discharge internally because it is faulty, or there is still some electrical circuit left live after you think everything is switched off.
all batteries self discharge - but they shouldnt do it overnight or even over month. if you have time, the easiest way to check is to disconnect your battery at the terminals when fully charged, measure its voltage after resting a bit, and then measure it again after (say) a couple of days. if the voltage stays fairly steady at something like 12.7 v then the battery is not self discharging. if it drops from 12.7 to 12 or lower, its for the bin.
similarly, if you can disconnect one terminal whilst the bike is switched off and put an ammeter in circuit between the disconnected terminal and the wire that you have disconnected, you will see if there is anything eating up your amps overnight. if this is happening then you need your garage to find out what - but its better than them guessing its the battery and you having continuous problems after they have replaced it.
