You need a meter set to amps, remove the earth lead on the battery & connect the meter to the battery post & the other to the removed lead. Close everything down including the bonnet run the meter outside of the car, now lock the car & wait for at least 10mins for everything to close down. See if any currant is being drawn, if there is you need to start pulling fuses until you find the circuit that is drawing power!
DO NOT TRY STARTING THE CAR......you will fry the meter!!
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.