\documentclass{article}
\def\where{Unknown}

\begin{document}

\section{One}
Expect Section 1:\where.

\section{Two}
Expect Unknown: \where.
\begin{equation}
  \mbox{Equation 1} = \mbox{\where}
\end{equation}
Expect Unknown: \where.

\subsection{Two pt one}
Expect Subsection 2.1: \where

\subsection{Two point two}
Expect Unknown: \where.

\subsection{Two point three}\label{labelled}
Expect Labelled subsection: \where

\subsection{Two point four}
Expect Unknown: \where.

\section{Three}
Expect Section 3:\where.

\subsection{Three point one}
Expect Section 3: \where.

\section{Four}
Expect Unknown: \where.

\end{document}
