Smoke from xc60
 

Good evening .
I bought a xc60 auto, 15 plate with 33,000 on the clock, its mint condition.
It seems that every couple of weeks or so when the car is parked, it starts to smoke from under the hood, it has been to volvo twice and both times they can find no reason for this, they can find no faults on the system and nothing hot or melting under the hood.
Any idears ?

How soon after being parked does the smoke appear?When it does smoke what does it smell like[smoke from electrical fault smells different to smoke from hot oil burning off the exhaust for example]Is it definitely smoke and not steam[e.g. coolant/windscreen washer fluid/other water source]evaporating off a hot engine?
First thought was possibly alarm siren internal batteries leaking onto its circuit board causing a short of some kind,but I see the car is only 4yrs old so would think this unlikely plus the intermittent nature of the problem is a strange one.