The heated rear screen may work only when the engine is running, as often on some cars, more notably for front but some rear screens the relay is fed the live it needs from the IND terminal on the alternator(charge light).
It's possible the relay is duff, and actually powering the rear screen a bit all the time, not enough to make it appear working, but enough power sap overnight etc to make a big difference.
Unplug the rear screen relay and see what effect it has, as it's "not working" anyway this won't be any inconvenience.
Your charge rate is ace, a new battery will cope with being fully flattened and recharge ok, not so when a few years old.
To find a power drain, remove all fuses whilst you have a 5 watt bulb placed between the battery terminal and the lead that came off it, put each fuse back in turn, when something consumes power the bulb will light, you should not have anything more than a very faint glimmer, as some stuff uses all the time, clock/alarm etc.
tabetha