Hello, I hope the following assists you.
There are a couple of concerns that can cause the alarm to sound when the car is left.
The bonnet ajar switch can give problems, if this circuit becomes open then it is the same as the bonnet being opened, you can try to by pass the bonnet switch, this is part of the bonnet latch but if you disconnect the wires from the latch and bridge the connector see if this cures the concern, if it does then replace the bonnet latch.
There were issues with water getting into the alarm horn causing short circuits, this normally results in the alarm going off but more commonly the indicators will flash with no alarm sound. If this is the case the alarm horn is behind the left front wheel arch liner, disconnect the connector and see if this alters the concern.
Start with these points, let me know if you have more questions and I'll reply as soon as I'm able to.