It's been a while since I've had this problem. When I kept my Fujitsu lifebook in my backpack, it sometimes wakes up from suspend/hibernate mode. This happens in Ubuntu & Windows.
So it must be the mechanism of the notebook lid.
To disable it in Ubuntu, it's easy:
After chmod 666 /proc/acpi/wakeup
The command is: sudo echo "LID " > /proc/acpi/wakeup
It works. Now supposedly it should work automatically on boot by placing the command in /etc/rc.local