Mercurial, some useful info on your link: 150mA @ 17 volts.
Doesn't sound like much but even at half that, at this time of year with short days and limited sunshine, the solar panel could put out say 450mA per day based on six hrs of daylight. The voltage reduces significantly without bright sunshine but as long as there's ultra violet light, the panel will at least deliver some current and this should help to offset the drain current consumed by the alarm unit when it's in sleep mode.
This could mean the difference between being able to start the car, and not after an extended period of non use.
In a few days time, I'll be returning from being away and my car will be in sleep mode with the solar panel connected. What I need to do is take the panel out of the circuit and see if I can measure the battery voltage via the wires going to the lighter socket prior to waking up the alarm unit.
If I can, then Mark's "time-out" suggestion mentioned prior obvoiusly does not apply to my car.
If I can't, then the lighter socket - which on my car remains live when the ignition key is removed - will have powered down together with the other systems mentioned in the manual extract posted by Richard.
I'll report back the findings.