The 70 series have the siren the 850 had a horn, the alarm systems are completely different.
Often the 'false tripping' on the V70 is caused by frayed wiring to the tailgate. The rear screen heater and the two side screen wired grids are used as a 'break glass' detector on the 70 series alarm system. The three elements are wired in series.
If you go to the dealer or use the latest version of Vol-FCR the cause of the last alarm can be read from the VGLA unit, this will help diagnosis of the cause.
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