Looking at a particular proposal, the "edited by" section shows that a former staff member (Jeff Atwood) edited it. However, for some reason, a diamond appears next to his username:
Jeff Atwood was an employee, but left the company and his employee/mod status on every site was removed, so there shouldn't be a diamond displayed next to his name.
This can be replicated with different users, but I'm not sure if it's limited to former employees.