Modern cars with lots of electronics always have battery drainage even the engine is off. Electronics will go to sleep mode, but it will still consume a little. I have designed many electronic accessories and decided to check if there is any effect on battery life. My test car is the 2017 Mazda MX-5 RF Launch Edition. It has a SmartTOP module installed as well as my front camera kit, ACC controller, and automatic footwell light.
Measuring current accurately is a little tricky because any circuit interruption will wake up all electronics again. Also range from 10A to milliamps. To avoid these issues, a circuit switch is really handy. I found a good option on Amazon. Because I’m often testing new electronics, I found that having this device installed is time-saving.
Power on 
Power off
Now, when the switch is in position “Power on” I can safely attach the multimeter and after 2 minutes switch can be disconnected and a small current will go through the tester.
The proximity of Key Fob seems to have no effect on this test, however when the car was locked, this current drop to about 15mA, which is a very little drain.
This means if the car is unlocked for a long time, it will drain the battery faster.
Gadgets for my MX-5 Miata (ND) 
I found this to be very interesting, Sergey. So I don’t need to unplug the OBD2 courtesy light module during the winter? Just lock the doors instead?
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OBD light module will be in deep sleep, however, for winter it is better just disconnect the battery
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A good experiment would be to test with the hood open, as I think I read somewhere that the car doesn’t go into sleep mode, if you have the security alarm system. This is usually done while a trickle charger is attached, but of course, the charger will more than make up for the dark current drain.
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I did my test with the hood open, as you can see in the photos.
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I guess your hood latch doesn’t have a switch.
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I did something similar when I first got my 2016. One thing I discovered with the car off is the 100ma or so current would drop down in stages after about 30 minutes or so. It finally would get to around 10ma as various modules went to sleep. If I opened a door or the trunk it would wake up for a short time and then go back to sleep. It did not matter if I had the key fob in the car or not and the whole time I had the hood open. I don’t remember if it made any difference with the car locked or not.
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probably locking the car makes some modules go sleep faster. I tested only for 10 min.
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