Always carry valuables including cash on you during all overnight transport. Even if you think it is safe in a hidden compartment in your combination locked back.
From Krabi to Chiang Mai, 2 bus rides later:
My bag still locked, my secret compartment still zipped up, my cash wallet lay inside, unzipped and coins scattered everywhere. My notes were gone.
I assume a person was in the under carriage during the trip and was a professional in picking locks. Stole about AU $400 worth of Euro, Aussie Dollars and US Dollars. Learnt that lesson pretty quick.
Don’t have anything valuable in your bags if you cannot see where your bags are at all times.