To try to locate the problem, could you take the bike for a run of 5-10 miles, disconnect the battery,and then take a voltage reading after the bike has stood for a few minutes. Take another voltage reading after it's stood for a day, and then after 2 days. If the voltage has reduced substantially during that time, then it's probable the battery is faulty. If, however, the voltage stays up while the battery is left disconnected, then it's more likely that something on the bike is draining the battery.
If you liken electricity to water, the voltmeter is measuring how much "pressure" there is; an ammeter will measure how much is flowing, just like a domestic water meter.
If your water pipes leak then water will be flowing through your water meter even when all the taps are closed.
It could be that a component on your bike is "leaking" current. When everything is turned off there should be no current flowing. Turn your multimeter to ammeter position, disconnect one of the battery leads, and use the multimeter to bridge the gap between batter and lead. Any current then has to go through the multimeter. There shouldn't be any, but if the meter shows some current it reveals that one of your bike components is leaking current.
Merda taurorum animas conturbit.
"Let's ride motorcycles!"