If you last locked the car with the key only, then I`m betting the alarm will not sound.
Wrong.

Either way as soon as the car is lifted that too will trigger the alarm.
I proved that to myself when I started jacking up the car witout first unlocking with the key fob button. 
It must've been a fluke, because there's no tilt alarm on an Omega

this what my handbook says regarding tilt alarm
Switching system on excluding
moni toring of the passenger
compartment and the
vehicle tilte.g. if animals are to be left in the vehicle.
1. Close boot lid/tailgate and bonnet.
2. Press button Ä. LED flashes (for a
maximum of 10 seconds); see page 61.
3. Close doors.
4. Switch on Vauxhall alarm system. LED
lights up. After approx. 10 seconds the
system is activated, without monitoring
of the passenger compartment or vehicle
tilt. LED flashes until system is switched
off.
6
re arming the alarm
To activate:
All doors, windows and sun roof 3 must be
closed; press button p on the remote
control unit again within 10 seconds after
locking
– or –
turn key in d iver's door lock towards rear
of vehicle again within 10 seconds after
locking, then turn it back to the vertical
position and remove.