In addition to checking ground connections per the other good comment chain, many DVMs (digital voltage multimeters) include a current test feature, including cheap ones from harbor freight.

Instead of measuring the system voltage, you can see how many electrons per second are going in, or out, of the battery in terms of amps, very useful when you need to see if something is sucking battery even when the bike is supposed to be “off”.

On my fluke meter, you switch the knob over to the A symbol that isn’t squiggly (farthest position in this particular model), as you’re measuring DC current. You’ll then unplug the red wire from the usual voltage position and plug it into the left-most A position.

After checking ground connections, while the bike is off and battery at least somewhat charged, disconnect the positive lead from the battery (leave negative connected). Now, jump the now disconnected positive lead back to the battery, you can do red dvm wire to positive bike wire, black dvm wire to battery’s positive terminal. Do not measure directly across battery while dvm leads are in this mode, it will look like a short circuit to the battery!

Anyway, the reading in the DVM should be low, something like 100ma at most. You’re measuring how much energy the bike computer etc is drawing from the battery while it’s supposed to be “off”. Anything more than 100mA indicates there’s something possibly wrong with ECU (less likely), wiring harness (less likely), or there’s an accessory that is poorly installed or poorly designed.

Good luck, if this doesn’t work, next step is starting the bike without dvm in current mode, and either check system voltage (should be about 14.4v when idling) or you can check the system current while the bike idles and at other rpms to identify whether electrons are going into or out of the battery.