I’ve heard this expression all my life and am astonished that hardly anyone else is familiar with it! Perhaps it’s a regional thing? I grew up in Texas and lived in Colorado for 12 years.
Here’s my purely speculative take on its possible origin. To be "out-of-pocket" is to be someplace other than your usual location—somplace where people can’t readily locate you, someplace where they can’t readily "put their hands on you," to use another idiom.
No connection at all to out-of-pocket expenses.
Again, this is all pure speculation on my part.
penny