@Hawthorn37
There are two sensors - the interior movement sensor & then there is a tilt sensor which is designed to trigger if the van is lifted or towed.
Its easy to check if the tilt sensor is turned off at home, just jack the van up on any corner.
I am guessing it is the tilt sensor that is triggering.
Looking at the T6 manual on line ( only one I can find) it states that if Safelock is activated by the remote control the anti theft alarm is still set - ie open a door / tilt the van & the alarm will still sound.
If safelock is set by using the key in the drivers door lock, by turning the key to the close position twice, then the anti theft alarm is disabled.
Assuming the same applies to T6.1 it would explain why you can use the double press, leave the dog in the van etc & it all appears to work correctly, but stick it on a ferry & the alarm sounds.
Further reading suggests that the old B pillar button also disabled the anti theft alarms.
Logic would therefore suggest that going via the menus to disable the Safelock will also disable the anti theft alarm as the menu option replaced the button, whereas the double press on the remote will not.
I have attached the 2 pages that I am reading from below, its not the whole manual.