My car battery seems to go flat quickly. If I do not use my car
for 2 weeks then it will not start. Also, if I leave it with the
tiny map light on, it goes flat overnight (Not even enough juice
left for one crank). What might be causing this?

I can't think of anything that would drain the battery while
the car is off, except for the alarm. I seem to have noticed that
it goes flat more quickly if I leave the alarm not-armed (but I
haven't scientifically checked this).

I tried measuring some voltages. After charging on the charger,
the battery was 13.25V . When my car is idling, it is 13.92V .
After leaving the car overnight, the voltage was 12.62V .
When idling after that. 13.92V. After a 5-minute idle and turning
the car off, it was back down to 12.65V.
While idling, the map light causes a voltage drop of 0.05V .

I haven't tried checking the voltage immediately after a
long drive, to see if my alternator is working properly
(I was going to but my multimeter battery ran out :/ )

The battery is of the smaller kind physically, although I
don't see why this ought to make it go flat any quicker.
The only devices I have are a 50W stereo, OEM lighting,
and a boost gauge.

The battery has had this go-flat-quickly problem ever since
I bought it.

The battery has a coloured spot on the top that's designed
to indicate whether it needs charging or not (green = charged,
black = flat, clear = needs water). The spot is a faint green
most of the time. It used to go clear as soon as I tilted the
battery in one direction by about 3 degrees, but I filled it
up a few weeks ago and it doesn't do that any more. (I'm not
sure how that works though, because the 6 cells are all separate
so how can it indicate them all?)

When I put the flat battery on the charger, it indicates 'fully
charged' on the charger for about 5 minutes, after which it resumes
normal operation. I haven't seen this effect with any other battery
I've charged. What does this mean?