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Get a meter. Unhook one of the battery cables, set meter to read amps, and place meter leads in between battery and cable you removed (one lead on battery, other lead on cable).
This will tell you how much current is constantly draining from your battery.
Now pull fuses for stuff and see if current reading drops dramatically after each one. Obviously start with alarm fuse.
This will quickly identify whuch circuit is using too much juice when car is parked.
Then you can either dig further into why, or just leave that circuit disconnected.
You can also try this, if you have an accurate volt-ohm meter. Measure the voltage across your battery. Note the voltage. Now remove the fuse from the alarm and see if the voltage changes. Voltage change of over 0.1 VDC or more is a lot in a 12-volt system. You need an precise DVM to do this correctly.
I have an old-school Fluke 87 that I use for all my voltage readings. Their $$$ --- but if you do other electrical work (like fixing guitars and amps) or working on house wiring... they are worth their weight in gold.
Even cars without aftermarket or dealer installed alarms will kill battery with just the normal parasitic draws from the radio memory and remote locking system. Once these have killed the battery a few times with severe drainage the battery will be damaged beyond repair.
Several Youtube videos on how to troubleshoot with just a cheap multi meter. Old girl friend had this problem and we finally determined the light in the trunk was not turning off when the lid was closed. Took a very willing small person in the trunk to confirm!
There is no normal thing that will drain a battery in a day or two on the car so you are correct this is not normal. If it sits for weeks, especially in cold then yes, the battery can get a bit low and a battery tender is recommended, but if this is happening in days, then definitely something to look at.
Disconnecting a battery can be a solution for long downtimes, but the battery can still discharge some disconnected and a properly maintained battery should last a bit longer as a tender will actually slightly drain and charge over time which is good for the battery. Plus then your radio settings, idle learn, etc do not have to be repeated which is nice.
For testing parasitic drain I recommend a current measurement vs voltage. A lot of affordable DVMs have an amperage setting. Set up the meter and start on the 10A or 1A side if it has one to make sure you dont blow the meter fuse if something higher current is on by accident. You remove a battery cable and connect the meter in series (one lead to the bat cable, one to the battery terminal) so the meter is completing the circuit. Wait a couple of minutes and make sure no dome lights or other lights are on and that the key is off. The reading will start higher as the ECU boots and goes through its initialization. I would have to dig it up in the shop manual, but I am guessing it will read up closer to an amp at first (probably 800mA to 1A) and then drop down to a much lower value. If it is reading much higher, then it is time to test. Start by removing power to the alarm if you suspect that. You will see the current reading drop and can figure out how much of it is that. Then you can proceed to pulling other fuses to see what changes. Recommend starting with things like radio circuit, lighting circuits first.
We just moved and all my shop manual stuff is in a box somewhere, but when I get to it I can update with the typical values you should see here
