and it was the gen3.5

Iv had my coupe (gen3) for about 6 months now, i always pressed the button in but after about a month i was getting out and heard a loud bang, looked back to discover the handbrake was no longer on! This happened twice more during the next few weeks.

I now apply handbrake without using the button and stick it in gear just to be safe. At the end of the day what will be cheaper, replacing a worn handbrake or repairing your bodywork/buying new car after it rolls away!!!

This is a problem on a number of cars. Vauxhall first discovered that the ratchet does not always engage if the button is depressed when pulling the handbrake on. It has a tendency to slip and let the handbrake off causing the car to roll away on inclines. The issue was discussed on the "Watchdog" program on BBC which is why new advice was stuck in handbooks to cover manufacturers in the event of an accident.
The same happened to Hyundai cars so they amended their book as well which now says do not push button in, use the ratchet. This way it always engages.
It does not cause any damage, just a little extra wear on the teeth of the ratchet and of course the noise of the spring loaded pall rattling over the teeth of the ratchet.
Driving instructors always insisted that you push the button in but this was to stop the noise and was more of an etiquette thing than a safety issue.
My coupé's an Auto so I use "PARK" to lock the gearbox and I don't use the handbrake.
If you have any doubts about your handbrake you should always turn your wheels towards the kerb so it can't roll away. Just remember to straighten them back up before you set off.
Uncle Buck