Hi,
Battery is only about 6 months old
Despite the fact battery is only 6 months old you may have a dodgy battery, just like anything else they can be faulty from new. If the alternator is capable of kicking out 14.2 + volts there is not much wrong with it. In the past I have seen an alternators that have 'lost a diode' in the diode pack and could not reach 14 volts ( something around 13.7 to 13.8 which will not charge a battery ).
I have never heard of a battery discharging back through alternator. If alternator is kicking out over 14.2 volts it should keep battery charged easily, but frequent starts and short journeys can run a battery down. You really need to get a voltmeter and check voltage at battery terminals with engine running - it should be around 14.2 to 14.5 volts, should let battery stand for an hour after engine has stopped ( to get rid of 'surface charge' which can artificially raise the terminal voltage ).
Alarms and other parasitic drains like the boot light or interior lights are most likely, a relay stuck in would drain the battery in a very short time, ( measured in hours not days ) relays are used to supply heavy loads - Aircon relay stuck would drain a full battery in about 10 hours, rear screen about 3 to 4 hours. Interior or boot light drawing 5 watts ( about 0.5 amps ) would take about 60 hours to drain a full battery.