Users missing from auto-complete list

As far as I'm aware I have no defined lists of users and I've been able to Masquerade as people for years.

I've recently upgraded loads of modules on my site and now I find there are some users which don't exist in the Masquerade auto-complete.

Should all users be available in this list?

What do I do if I can see users in the user list but can't find them in the autocomplete?