To make sure I've indexed what I should I want to footnote every indexed item in a draft of my document. The MNWE that follows is what I've tried. The renewed \index command properly creates an index entry, but doesn't generate the footnote. \marginpar instead of \footnote fails too. So does \marginnote.
\documentclass{article}
\usepackage{marginnote}
\usepackage{imakeidx} % needed in the real document
\usepackage{refcount} % needed in the real document
\let\oldindex\index
\renewcommand{\index}[1]{%
%\footnote{INDEXING #1}
\marginnote{#1}
\oldindex{#1}
}
\makeindex[intoc] % needs imakeidx
\begin{document}
Here is an entry\index{entry} to index\index{index}.
Check that margin notes work\marginnote{This is a margin note}
Check that footnotes work\footnote{This is a footnote}
\printindex
\end{document}

egregbecause you misusedindexan his package. Please consider, that\indexhas an optional argument here, which is forgotten with\lethere – Sep 13 '15 at 15:45