In standby (all off, doors closed) only radio, clock and doorlock should consume power.
Example values for VIN3855: Current loss 43mA (10mA central locking, Clock 7mA, Kenwood radio 26mA). Battery Voltage: 12.3V. Voltage drop in standby: 0.2V.
For diagnosis, measure the current (Amperes) or voltage (Volts) at the battery between the positive (disconnect from the battery) and the battery positive terminal. To exclude a consumer, remove the corresponding fuse in the fuse box
For saving current in standby you can read in the Forums about corresponding power saving circuits for clock and doorlock module.