There will always be a small amount of residual current for things like the alarm and the central locking receiver typically around 50mA .
19 days is actually quite a time to leave a car with a typical drain of 50mA being 0.05A , Multiply that by the number of hours in 19 days
0.05 * 19 * 24 = 22.8Ah of capacity thats nearly a third of the capacity of a fully charged new battery.
Extra kit such as non standard radios, amps etc are probably going to increase this.
What make is that battery that you have fitted?, a lot of the aftermarket ones don't have the performance of Varta, Bosch OE batteries.
Karl.