Hi there,
I have been dealing with these issues for the past 20 years, first with my 1988 Hyundai and now with my Volvo v70 1998 for the past 5 years. I might be able to shed some light on what happens.
FIrst, a 40AHr battery will have roughly 1.5AHr self discharge per day. 15ma is 0.36AHr per day, which is about 25 percent of 1.5AHr.
That means that the 15ma, after 4 days, equals 1 day at the normal self discharge, so at the end of 4 days you have a discharge of about 7.5AHr, and that is about 15 percent of a 50AHr battery, which if the battery was fully charged to begin with would take it down to about 85 percent of full charge. If that was 8 days it would be down more than that, around 70 percent.
This all means it would be good to know the number of days the car is being parked without starting. It also means knowing the age of the battery and if there was any misuse. These things cause the battery to drain faster. Also, knowing the normal charge current by direct measurement.
The charge current and charge time have to be long enough to get the battery back up to full charge or nearly so. If not the battery will drain little by little until one day it is seriously low and may not start the car.
I've ran into this so much in the past with this car and other cars i decided to install some telemetry to measure the battery voltage 24 hours a day 7 days a week. This measures the battery voltage all day and night and sends the data to a receiver in the house where my computer logs the voltage for every single measurement. If it goes low, i know it almost immediately. That's the only way i think it should be done.
With this setup, i found that the battery never charges all the way even if i take a ride across town. I don't know how long of a drive it would take, but i can't go driving around town just to keep the battery charged, that would be nuts. This led me to install a solar panel battery charger. Now the battery voltage is probably higher than any other car in the town. It's not as good on cloudy days, but it averages out to something much better than nothing at all. This of course means i have to recommend it to anyone having this problem.
It's difficult to pin down exactly what is happening sometimes because the battery condition has a lot to do with it too, and it could happen even after 6 months after installing a new battery. I for one don't want to have to keep buying new $200 batteries every 6 to 12 months.
I found that a 20watt solar panel mounted on the roof of the car works ok. Anything lower might not be enough. You can go higher, but you have to make sure it comes with a charge controller. This is not a max power tracker either, which you don't need, only a charge controller. That limits the max voltage of the battery to 14 volts during solar charging times. That's mandatory because you don't want to overcharge either. Many solar panels now around 20 to 100 watts or so come with a charge controller (not max power tracker) so all you have to do is find one that has that too. You also need a bracket to mount the panel on the roof of the car, which probably wont come with it, although a few did in the past. Beware though some come with a very cheap bracket that is only a bent piece of thin aluminum bar and that does not work at all. The good ones have two brackets, one for each side, and allow mounting at an angle of at least 45 degrees, although that angle would vary depending on where you live. You want the panel aimed at the sun for most of the day, or nearly so anyway. You only have to adjust for the altitude, you don't really need to adjust the azimuth although there are trackers that will track that angle too. In my experience now though, you don't need that just the altitude adjustment, and adjust it once.
This probably works with an aged battery too because the main difference between the old and new batteries is the self discharge, and if you can make up for that when charging, the battery starts the car ok. The solar panel will keep the battery going longer.
What i am not sure of is if there is any commercially made telemetry for the automobile that is not too expensive and you can install yourself. I could not find anything to my liking so i designed everything from the ground up and built it up with parts available at regular electronic parts outlets. Total cost under $100 USD not including the PC Windows based computer in the house used to log and display measurements. I don't know how hard it would be to build another one though there is a bit of soldering and stuff required.
Hope this helps.