I've opened the boot without unlocking before, and it's pretty noticeable that you haven't unlocked it, as the alarm's going "EEE-UUU-EEE-UUU", louder than you can imagine!
If you can open the boot without the alarm going off then you might want to check whether the alarm's working at all.
Not all of us have alarms on our Croma and mine is one of them. Mine has only remote door and boot lock/unlock. And I don't say things that I don't know or experienced so ...yes, I have unlocked it ..many, many times. It's what I always unlock when I am aproaching the car with some bags. When I click on unlock boot, the turn signals blink one time and after I close it (without touching the remote again) the signals blink two times and the boot auto locks up. This action is also described in the user manual if you read it, but I quote you "it's pretty noticeable that you haven't read it"
Now, if your alarm goes EEE-UUU-EEE-UUU when opening the boot, there is something wrong because in the manual says that alarm shouldn't trigger
I quote from the User Manual:
"Opening the tailgate
by the remote control
Press button Rto open the tailgate by
remote control even if the alarm (where
provided) is on.
Opening the tailgate is accompanied by the
direction indicators flashing twice; closing
is accompanied by a single flash (only
if the alarm is on).
If the alarm is on, when the tailgate is
opened the alarm system switches off volumetric
protection and the tailgate
perimetral protection sensor.
When closing the tailgate again, volumetric
and perimetral protection sensors are
restored.
IMPORTANT If the remote control does
not work properly, it is however possible
to open the tailgate through the mechanical
lever set on the lock inside the
boot (see paragraph “Boot” in this section)."