There are 227 "secret" routes in Connecticut: highways with numbers above 400.
"Secret" is just an unofficial term for unsignposted state roads and state service roads. Many of these are small auxiliary roads, or even long exit ramps.
The general public should never hear about secret routes, but they sometimes show up on maps, and a few road signs have even gone up.
This page discusses some of the more interesting routes in the 600s. These routes are mainly in DOT District 2: eastern Connecticut.