Go back to basics, check loading on all individual circuits and add the Ampère values up.
These should match the loading on the phase(s) and neutral.
Turn main isolator switch off, after the meter. The meter should stop. ( chances of a mechanical meter being faulty are remote, although electronic meters I wouldn't trust for erratic readings ).
Then one by one unplug or switch off the appliances. It can be a mammoth task, but if you want to check where the power goes, just do it properly.
Bear in mind that a lot of appliances are still on standby when so called turned off, best is to pull the plug from the socket.
Now these days there are a lot off plug in wallwart and SMPS supplies drawing a couple of watts each 24/7, this adds to a reasonable base load.
And sometimes people complain but they forget how long appliances or lights are left on for sometimes.
