With everything turned off, doors shut etc, half an amp pull is far too much.

Batteries are ever decreasing circles of power.
A 60 amp battery won't last 60 hours with a 1 amp pull as after an hour it ain't a 60 amp battery anymore!

There will be a little drain on the battery when left alone, radio presets, clock etc all pull a little juice, but it should be very little
(some where nearer 0.04 or 0.05amps or less)

With a cheap digimeter set to read amps and a fuse diagram, you could pinpoint the drain quite easily yourself.

With the bonnet up, window open (in case you need to lean in), ignition off, doors closed and everything turned off.
Pull a lead off the battery and bridge the gap with the digimeter.
Then start removing and refitting the fuses one at a time while recording the amp reading at each stage.

When you pull the right fuse the pull should drop considerably and you'll know what circuit the drain is on.

The usual culprits for this sort of drain are aftermarket head units.
They get wired in incorrectly, the switched live being connected to a permanent live, this tends to pull half an amp or so even with the unit off and will flatten a battery enough to stop it starting in less than 24 hours.