I was really surprised while reading this thread that no one mentioned the primary use of 'out of pocket' that I know.
Examples:
'John has paid for everyone's drinks on his own; he's out of pocket so let's pay for his taxi home.'
'Since starting to pay my student loan off I'm a couple of hundred Euros(!) out of pocket every month.'

As for the absent/away meaning,.. very dodgy! I like Island Pete's reasoning as to why (i.e. in/out of someone's pocket), but I would, nevertheless, not touch it with a barge pole!