[ Quoting <mikachu@gmail.com> at 14:57 on Dec 18 in "Re: default to file ..." ]
> "scan" is a command that already has completion in zsh (it's part of
> mh), but if you don't have mh it won't be able to produce any
> completions. To disable a bundled completion, run compdef -d scan
> after running compinit.

Ah, that explains things:

% mv scan 1scan
% ./1scan -zone <TAB> (works)

Thanks for your reply. I'll just rename this command.

 grtz,

-- 
    Miek