Alarm in an older car (2005 Volvo V70) goes off almost every time a mobile modem and WiFi hotspot is left in it (Huawei Mobile WiFi E5377).
This happens even if:
- the modem is switched off
- the battery is removed
- the battery is removed while the modem is switched on to drain any internal capacitors
If the modem is left in the car, it usually takes from a few to a few dozen minutes for the alarm to go off. If it goes off, unlocking and locking the car does not guarantee that the alarm won't go off again.
What could cause this and how to prevent it?
Update 2019-05: The problem mostly went away on its own, although it still occurs every few months or so. Car service tried to figure it out, but couldn't find anything wrong. They suspected a possible gap on the trunk door, but fixing that did not change anything at the time.
Update 2023-09: Coming back a "few" years later - the car is still in use, and the spurious alarms haven't returned. The modem is still available, will give it a go just for the curiosity.