What is the time scale between parking up and trying to start the vehicle again?
If it's weeks, then yes, these days with small drains from alarms etc might drain the battery. If it's less than two weeks then I'd say you have a problem. If it's more than three or four weeks then a 'smart' charger can be plugged in to keep the battery topped up (I use one on my motorbike in the winter when I'm not out on it as much as the summer).
To to try to eliminate the problem is fairly simple by clamping an amp meter to the main positive cable on the battery and seeing the reading. Straight away this will tell you if you have a high drain with everything turned off. If it's high, then unplug each fuse in turn to determin which circuit is pulling the drain. Then work out whats pulling off that fuse.
To answer your next question, no I don't really know what would be an acceptable 'low drain' from such things as the alarm or perminant live to the radio (to keep the stations memory), but it shouldn't be much.
