Sounds like you have a very small discharge probably all the time. If you have access to finely calibrated ampmeter disconnect pos terminal and connect ampmeter in series to it. If discharge is present should show it. Disconnect wiring loom in stages to isolate short-ie. when meter goes back to o will be in the wiring that you have just disconnected. This is with the bike at rest and no lights switched on. Have used this many times to isolate this type of problem. Hope this helps-the ammount of discharge can be just a trickle even milliamps but will drag your battery down. Obviously the longer it sits the worse it gets. Voltage can be misleading as the charging amps can still be under the spec as I discovered as a apprentice many decades ago.
