I have a 2005 Fortuner and it recently went into the shop to have a few scratches and dings repaired, and since it came back the alarm has been randomly going off at all hours. I can only guess that one of the geniuses at the repair shop somehow adjusted the sensitivity, or is it something else, like an old battery?

It seems this might be a Fortuner thing. While I was out shopping today, I could hear my car alarm going off again so raced to the parking lot to turn it off, only to discover it was another Fortuner making the noise. The plot thickens.

2005 'tuna?

It's their 'you need a new car' alarm.

Can only be stopped when you no longer own it (trade in or whatever).

Same here, 2006 Fortuner. Ours also started doing this after a service, that was 1 or 2 years ago. We tried to have it fixed twice but nobody knows how to do that and it's very much intermittent. Sometimes it's fine the whole night, sometimes not.

We found a simple solution which is to lock it with the key rather than the remote. Can still use the remote to unlock of course.

To be honest if I could be bothered I'd simply have the shop disable the alarm altogether. Then I could once again use the remote to lock the car... car alarms are not very useful after all. Either you live in a place where you absolutely don't need it, like Thailand. Or you live in a place where you might need it but where car thieves get around the system easily.

Either way the only feature your car alarm provides is to go off when it shouldn't.

After a service at the main dealer and this happen to me, I would make the time to keep taking the dam thing back to Toyota until they do fix the fligging thing.

Does it have a hood sensor?

In my case the hood sensor was a bit "sticky" so I adjusted it a bit and the alarm was back to normal.

Some cars disable the motion sensor if you press the "lock" twice. Worth a try.