When the -O option is used to control an ssh process over the master
socket, it is not affected by quiet mode:
chrisb@telly:~$ ssh -M -S test.ctl -f -N pie
chrisb@telly:~$ ssh -q -S test.ctl -O exit pie
Exit request sent.
The manual defines quiet mode as:
-q Quiet mode. Causes all warning and diagnostic messages to
be suppressed. Only fatal errors are displayed. If a second -q is
given then even fatal errors are suppressed, except for those
produced due solely to bad arguments.
The above message is clearly not a fatal error, but is displayed anyway.