Just guessing, but a battery inside a computer/phone, even if turned off is probably still powering at the very least a few wires. Even if they don't go anywhere (i.e. they are not connected anywhere else) they probably are slowly eating away the battery. On the other hand, I don't know how modern computers/phones turn off, but I doubt they involve a full circuit cut (i.e. there is no big red button making a click sound and closing all wires coming out of the battery)

From what i found with my phone, I would like to confirm that even if phone is turned off, power is still supplied to few wires. When i remove battery from my phone and then put it back and turn it on, it would ask me to configure date settings. But if I just turn the phone off and then turn it on, it doesn't prompt me to configure date settings. so I can tell that at least the clock on my phone uses battery even when the phone is turned off.

Two reasons:

1. Battery leakage. You can't charge a battery and leave it somewhere disconnected and expect to come back to a full charge. Small amounts of power leak all the time. Look for "self discharge". There are lines of batteries (particularly Sanyo Enelopp) that are marketed as low self discharge i.e. they can be stored charged and will still be usable in time. There is still a high resistance when something is connected which is enough to help with self discharge.

2. Most phones have a non mechanical on/off switch so a tiny bit of current is still used to have soft on/off functionality and watchdog circuitry online.

Thats one reason I nearly never use my smartphone.

My old Nokia dumpphone has a battery life of more then 2 weeks. But the Motorola Defy+ will eat a full battery even after shutdown within a week. So it does not even make sense as a water proof camera for sailing.

In result: The Defy+ is eating more battery when powered down, then my old Nokia when powered up. Imho, thats enough power to say "here I am" regularly.

Does your laptop support Wake-on-LAN[1]? I know I have it activated (and sometimes use it), which means though that it doesn't power down 100%, since the NIC still has to be reachable by magic packets. That's just one reason why it might still drain battery.

[1]: https://en.wikipedia.org/wiki/Wake-on-LAN

Oh, didn't even know that existed. When I fire up my laptop I'll take a look. Cheers.