pochemunyet,
I've also heard that claim made before. So,...I asked my broker the same question several years ago.
He said that having a standing sell order offers you no protection from the brokerage selling the stock short. I expect that policy to remain the case today. I suppose something can be negotiated to prevent that occurrance but even then, how can you ever prove it if they did permit a 3rd party to sell your shares short or have them hypothecated? Best I can imagine is you'd probably have to sue them with some kind of Forensic Accountant hired for your discovery - a truly expensive endeavour - and what are your damages assuming a successful Judgment in your favor? You get nothing out of this.
Just in case you are curious, I am with C. Schwab (not to be confused with K. Schwab, the WEF dude).
BOL,
WISEGUY