What exactly is standby mode?

I would not leave the notebook in standby in a backpack...the computer is still barely "alive" in standyby and can startup inside your bag without your knowledge.
This has happened to me before. I small push or perhaps something pushing the CD drive button can sometimes turn the laptop come on and it can overheat quite easily in your bag, as well as run your battery out without your knowledge.

The difference between hibernation and standby is that in standby the memory is kept in RAM (volital memory) and the processor and ram are still powered to a small degree to keep the info and await some action to restart the pc. In hibernation the RAM contents are written to the hard disk and the computer turns almost all the way off (though not quite, usually there is a blinking LED or whatnot).
There is minimal power usage in standby and the laptop can remain in standby for days without being plugged in while in standby or hibernation.

In either case turning off the computer is better because it cleans out any garbage left in the ram, and it makes the CPU safer for transport. As far as overnight, I either lock the computer (WINDOW+L) to keep my GF from seeing what naughty sites I've to, or turn it off completely.

I know the techincal explaination is kinda basic and even somewhat questionable, but the risk is