Package thmbox seems to add an extra space at the beginning of the body of the labeled theorems. See example below. Package thmtools with the thmbox option does the same. Latex original, package amsthm and package ntheorem do not add the extra space. I can solve the problem that occurs with thmbox by placing a % just after the label. However, I do not want to do that because I should edit each already written theorem. Problem is solved by including \ignorespaces in option bodystyle of thmbox. However, then the body is indented. If bodystyle is defined using \noindent\ignorespaces then the body is not indented, but the extra space shows up again.
\documentclass{report}
% ----- no extra space
%\newtheorem{theorem}{Theorem}
%\usepackage{amsthm}
%\newtheorem{theorem}{Theorem}
%\usepackage{ntheorem}
%\newtheorem{theorem}{Theorem}
% ----- extra space
\usepackage{thmbox}
\newtheorem{theorem}{Theorem}
%\usepackage{thmtools}
%\declaretheorem[thmbox=M]{theorem}
\begin{document}
\begin{theorem}\label{L1}
With label.
\end{theorem}
\begin{theorem}
Without label.
\end{theorem}
\begin{theorem} \label{L2}%
With label, but solved, even having space characters here and there.
\end{theorem}
\end{document}
thmboxor packagethmtoolswith optionthmbox, without editing each already written labeled theorem. – ASdeL Jan 10 '12 at 20:11\label{}unless one uses\label{}%(trailing%added). Interestingly, even thethmboxdocumentation shows the use as\label{}%. – Peter Grill Jan 11 '12 at 03:13amsthmis used, but not withntheorem. Withamsthm, there appears to be an additional space before the3inTheorem 3. – Peter Grill Jan 11 '12 at 03:18