The issue of the car battery draining out is very common in cars that are parked for a few days, especially with an active GPS system.
Typically, the GPS device draws around 50 milliamps for working under normal conditions where the GSM signals are strong. The battery drain becomes multifold when the car is parked in a low signal strength location. This is because when the signal strength is low the GPS device tries to connect itself with a GSM tower in the vicinity and for this, it draws a lot of power trying to get in touch (searching for network) with any of the various cell towers around the location. (You may have noticed how your mobile phone battery gets drained very fast when you are traveling by train through remote villages where the GSM concentration is low. This is very similar to the situation of a car parked in a low signal area or basement parking.)
Apart from the GPS device, there are other sources of battery drain in a car such as:
Car Stereos:
In standby, they'll draw only a few milliamps. But if they fail to go into standby, or if the DIP switches on the amp are set incorrectly, they can draw as many as several hundred milliamps, even though they're not producing any actual noise or music.
Remote controls & proximity keys:
There's a radio receiver that continuously listens for the key's frequency. When the receiver hears a signal at its assigned frequency, it wakes up to see if the key is the one that matches the car. That draws more current for a minute or two until the receiver abandons the possibility that it's about to unlock the door for the master. This might be an issue if you leave the car parked for many weeks without starting it. Imagine the confusion of a car parked near the elevator door in a busy parking structure. Every proximity key that walks past makes it sit up and beg, draining your battery for a few minutes. Soon, dead battery.
This issue is even more profound in the case of hybrids like the Toyota Prius. The key receiver operates on 12 volts, as do most of the accessories. More importantly, the 12-volt battery operates the main computer that controls everything else, like the door locks. The Prius starter, however, operates off the 280-volt traction battery—but the main computer has to be powered up for anything to happen. Since it doesn't have to start the engine, the 12-volt battery of the Prius is very small, 38 ampere-hours. So while a 64-ampere-hour battery in a normal car can last for weeks of a proximity receiver checking for the right key, the Prius battery can run too low within a few days. Bottom line: Turn off the proximity-key function, from the Prius's electronic dashboard, whenever you're parking it longer than overnight. It might be possible to do this on other vehicles as well. Check the owner's manual.
Apart from this, other faulty circuits, relays, etc also can cause drains.
When all the above work together, the battery drain can be considerable and noticeable. It is advised to start the car and drive a couple of kilometers every 3 or 4 days to avoid this, preferably without turning on the air-conditioning or the headlamps.