An abnormal parasitic drain could be a glovebox or luggage compartment light that remains on but undetected. Or an electronic component may malfunction and cause a parasitic drain that is larger than specification.

Even when the parasitic drain is small, over time the battery grows steadily weaker. 

In addition to parasitic loads, a certain amount of self-discharge is always occurring as a result of internal chemical reactions even when the battery is not connected.

Ambient Temperature Effects:
Temperatures above 77F (25C) increase the battery's internal self discharge. If the temperature is an average of  90F (32C), an additional 5% to 10% of the available amp-hours will be lost in a month due to self-discharge inside the battery 

When the temperature falls to 32F (zero C), the battery will furnish only about 85% of its normal starting power and the engine may need as much as 165% of its usual power to start.

Discharged batteries can freeze at 32F (zero C) and below which will cause permanent battery damage.
Other permanent damage may be caused by allowing batteries to stand discharged for extended periods.

Battery Parasitic Drain and Effects of Temperatures

I am not a battery expert. The following items come from my own years of experience with automobile batteries and marine batteries, plus much guidance from vehicle manufacturers and battery manufacturers. Some of these are quotes from various experts in their respective fields, arranged my me in proper sequence.  
Quotation from: The Tech writer Sleepy Gomez:

“I'll pass along some information from an interview I had with Gale Kimbrough,
technical services manager for Interstate Batteries ... . He considers a battery
to be fully charged at a level of 12.66 volts to 12.75 volts. A battery at 11.90
volts at no load is considered to be a fully discharged battery. Remember, this is
no load.”

“Watch That Battery. Batteries can be sneaky. A charged battery can show a full
charge when checked, but batteries usually fail slowly and lose the capacity to
hold a charge for a period of time. You need to have access to an accurate digital
DC voltmeter. Check the battery every day for several days. A good battery will lose
very little charge, maybe from a full charge of 12.75 to 12.73 in a week.
A battery that is going away might drop to 12.25 in the same period of time.
Although this battery is not discharged at this point, it has lost much of its
capacity and is dying. This is the same as having a bucket with a cup full of water
in it. When you pour it on your best friend, he will get wet-but not as wet as if
the bucket were full. How mad he gets is a measure of how good a friend he is.
Many starters have been replaced when the problem was a battery that was not able
to take or hold enough charge. For dependability, if a battery loses more than
2 percent of its charge in a week, I would not trust it. The same battery might
last a while longer in your daily driver because it is being charged every day.”
-Sleepy Gomez

This page contains some text that is freely available to the general public and it is used here under the Fair Use provisions of the US Copyright law.

WARNING: Batteries are dangerous.
Read my warning at the top of page one.

In automotive terms,a parasitic drain is an electrical load that draws current from the battery when the ignition is turned off. Things like the PCM (computer) and the radio memory are intended to draw a very small amount continuously. These draws are measured in milliamps (mA).
Why you might want a Battery Maintainer.
The next page is about battery usage in your vehicle.
Page 3.
Page 3 of 4.