Hi,

Zsh 5.3.1 error message < XXX not found > is printed to stdout instead of stderr.
Zsh 5.3.1 - 0 #> which ccache
ccache not found
Zsh 5.3.1 - 1 #> which ccache 2> /dev/null
Zsh 5.3.1 - 2 #> which which

This has been also tested with a fresh install of zsh (5.0.8)
Zsh 5.0.8 - 0 #> which ccache
/usr/bin/which: no ccache in (...)
Zsh 5.0.8 - 1 #> unalias which
Zsh 5.0.8 - 2 #> which which
which: shell built-in command
Zsh 5.0.8 - 3 #> which ccache
ccache not found
Zsh 5.0.8 - 4 #> which ccache 2> /dev/null
Zsh 5.0.8 - 5 #> which which