"Passive DNS" is a thing people sell. If people connect to your systems and use public DNS servers, chances are I can "map the complete endpoints of your infrastructure" without touching that infrastructure for a small cost.
If client X doesn't want client-x.your-firm.example to show up by all means obtain a *.your-firm.example wildcard and let them use any codename they like - but know that they're going to name their site client-x.your-firm.example, because in fact they don't properly protect such "secrets".
"Blue Harvest" is what secrets look like. Or "Argo" (the actual event not the movie, although real life is much less dramatic, they didn't get chased by people with guns, they got waved through by sleepy airport guards, it's scary anyway but an audience can't tell that).
What you're talking about is like my employer's data centre being "unlisted" and having no logo on the signs. Google finds it with an obvious search, it's not actually secret.
If client X doesn't want client-x.your-firm.example to show up by all means obtain a *.your-firm.example wildcard and let them use any codename they like - but know that they're going to name their site client-x.your-firm.example, because in fact they don't properly protect such "secrets".
"Blue Harvest" is what secrets look like. Or "Argo" (the actual event not the movie, although real life is much less dramatic, they didn't get chased by people with guns, they got waved through by sleepy airport guards, it's scary anyway but an audience can't tell that).
What you're talking about is like my employer's data centre being "unlisted" and having no logo on the signs. Google finds it with an obvious search, it's not actually secret.