\documentclass{article}
\usepackage{tikz}
\begin{document}
\newbox\TESTBOX
\setbox\TESTBOX\hbox{ABC}
\the\wd\TESTBOX % 21.8056pt
\box\TESTBOX % "ABC"
\bigskip
\begin{tikzpicture}
\setbox\TESTBOX\hbox{ABC}
\node{\the\wd\TESTBOX}; % 0.0pt
\node at (3,0) {\box\TESTBOX} % nothing
\end{tikzpicture}
\bigskip
\setbox\TESTBOX\hbox{ABC}
\the\wd\TESTBOX % 21.8056pt
\box\TESTBOX % "ABC"
\end{document}