Acceptable battery voltage drop overnight

That is not an acceptable voltage drop. If the shop did a full battery test and the battery tests good, you've got parasitic draw. Have it checked and fixed because this will eventually destroy your battery and it puts extra load on the alternator

Given your driving history, I would guess this isn't a "parasitic draw" problem at all. If you have had the battery continually undercharged for 6 months, it may have lost a lot of capacity.

If you measure the voltage immediately after the end of a drive, it should be more like 13.2, and drop to 12.7 within a few minutes as the chemical reactions in the battery slow down and stop when you stop charging it.

I suspect you are getting 12.7 immediately after switching off the engine and it drops to 12.1 in the same time scale.

If you want to check this theory, connect your meter while the engine is still running and leave it connected, then switch off and read the voltage every 5 or 10 minutes until it stabilizes. I expect you will see an immediate drop from 14.4 to 12.7 (not to 13.2 as for a good battery) and then a drop to 12.1 - not gradually over the next 12 hours.

With the battery in that poor condition, the regulated 14.4V from the car alternator will be barely enough to charge it at all. You might be able to get some more life out of it by disconnecting it from the car electronics and putting it on a high power charger that can apply 15 or 16V if necessary to force a reasonable charging current (5 to 10 amps) through it for a few hours.

Alternatively just write this off to experience, get a new battery, and don't abuse it for 6 months!

The parasitic current drawn while engine has stopped can be easily detected with the help of a general purpose multimeter (upto 10-20 amp dc).

Carefully use the ampere mode in the multimeter by selecting correct probe cables and selector knob. Disconnect the car's negative terminal. Now bridge the multimeter wires between the negative battery node and the disconnected terminal. Note down the ampere passing thru multimeter. If it is approximately 0.05Amp then there is no parasitic draw. Otherwise, if it is above this value there is certainly an error in some circuit.

This could be fixed with using elimination process by removing fuses step by step. The point when the culprit circuit fuse is removed we see a significant drop in amperage drawn thru multimeter. This circuit can easily be repaired for shorts or other problems.

P.S. Close all the doors, remove car ignition key, all the car accessories should be off. Only engine bay should be open. Waiting for 3-5 minutes is advisable so that all the automatic features stop working before checking the amperage.

Caution: Connecting multimeter with load above the rated amperage value of the multimeter can damage it.

I have a load tester that will tell the voltage of the battery. You should have the battery fully charged. You can put the battery under load test and it should return back to the proper voltage in the green range after the load test. You can get it tested like this with these at most repair shops. I have just worked on a battery that was only 7 months old and it will take a charge at 10 amps 14.8 volts for two hours and was fully charged. I let it sit out of the car for 5 hours and it dropped to 10 volts. The amps were so low that it had a hard time turning on the load tester. This is a very bad battery even though it is new. Even a low 1.5 amp charge would make the battery sound like bacon sizzling as it was bubbling so violently in one of the cells. You should only have small bubbles even at 40 amp charge. 12.1v is not barely charged? A battery should turn over an engine with a voltage of 10.9 volts if you have amps. I had a battery that was 10 years old that could start a v8 engine with only 11 volts. If you can not start your car with the volts at 12.1 then you have an amp problem (possible sulfated plates)