That sounds odd to me too, but kneeling respectfully before the google and asking politely:
To leave your designated area or be out of range.
Can you cover for me? I'm gonna be out of pocket for a whle.
A: I've always thought that the phrase "out of pocket" referred to the absence of money, not the absence of people. But the expression seems to have evolved into meaning unavailable. Why? I find it annoying!

Q: There appear to be at least three distinct meanings for "out of pocket":

(1) At a fiscal loss: In this case, "out of pocket" means out of one's own pocket—in other words, you have to pay for something yourself. Examples: "I thought the tickets would be free, but I got stuck with paying, so I'm $150 out of pocket." Or, "Mrs. Grosvenor refused to pay for the cabinetry, so the carpenter was out of pocket." The Oxford English Dictionary has this usage dating from 1679.

(2) Behaving badly: According to the newest edition of Cassell's Dictionary of Slang, "out of pocket" is a variation on the phrase "out of (the) pocket," a 1940s African-American expression referring to bad behavior or a bad situation. Cassell's says this meaning grew out of pool jargon (a shot that was "out of pocket" or "out of the pocket" caused a player to miss a turn).

(3) Unavailable: I first came across this meaning in the early 1980s when I was a staff editor at the New York Times. Reporters who had filed stories were supposed to supply phone numbers where they could be reached in case questions arose. If a reporter was unreachable (say, on a plane to Tibet), he or she was said to be "out of pocket." The OED cites published references for this meaning dating back to 1946, though it didn't become common until the 1970s.

I haven’t found an answer to your question about why the third meaning evolved. I also haven’t seen an explanation of why we say out of "pocket" rather than out of "hat" or "glove" or whatever when we’re unavailable, but here’s a possibility.

There's an expression "to have someone in your pocket," which means to have him under your control. Perhaps by extension, if he's "out of pocket" he's no longer under your control or scrutiny. Again, this is just speculation on my part.

Et par le pouvoir d’un mot Je recommence ma vie, Je suis né pour te connaître, Pour te nommer

And by the power of a single word I can begin my life again, I was born to know you, to name you

Paul Eluard