I'd definately be suspicious of a flat battery, for whatever reason
(i.e.
it's dying or perhaps the alternator is.
)
Your opening line is a very good starting point
-- you say you started the car, moved it around, and then parked up.
Starting the car is the hardest job the battery has to do and if the battery was low on charge then you probably used up the last bit of juice it had.
Normally when they're flat you can't start them
(I know, I know, too obvious.
.
.
) and you'll hear relays clicking.
You can also confirm this by checking your windows
(very slow in operation?) and perhaps your lights
(which may be very, very dim.
)
As it stands now, when you get it started next, take it for a good 30 minute run, on roads where you can do 40, 50+ mph
(and leave off power draining toys like the stereo, fan, and lights.
)
When you get back home you should have a well charged battery.
Turn off and try to re-start about 30 mins later.
If it doesn't want to know then either your battery has packed up or your alternator isn't charging properly.
That's a good starting point for you.
Regards,
Jonty.