Need itemize list items without using a list

This won't work if \xitem is in a list environment, but only at the "outer level":

\documentclass{article}
\usepackage{lipsum} % for the mock text

\newcommand{\xitem}{%
  \par\hangindent3em\hangafter0
  \noindent\llap{$\triangleright$\enspace}%
  \ignorespaces}

\begin{document}
\lipsum[1]

\xitem\lipsum[2]

\lipsum[3]
\end{document}

In order to have the symbol flush with the left margin, you can do

\newcommand{\xitem}{%
  \par\hangindent3em\hangafter1
  \noindent\makebox[3em][l]{$\triangleright$}%
  \ignorespaces}

Hanging indentation starts after one line, but the beginning of the first line is occupied by a box with the same width as the indent.

What about something like

\documentclass{article}
\usepackage{lipsum}
\begin{document}
\def\myitem{\hangindent=5em$\triangleright$}

\myitem \lipsum[1]

\myitem \lipsum[2]

\myitem \lipsum[3]

\lipsum[4]
\end{document}

The advantage of using \hangindent is it has a single paragraph scope so there is no effect after the item that needs to be "ended". The drawback is that it falls over if your "items" are more than 1 paragraph long.

This may be what you want, with help from @ChristianR and from
change the itemize from bullet to square

 \documentclass{article}

\newenvironment{notalistitem}
{\begin{itemize}
\renewcommand{\labelitemi}{$\triangleright$} 
\item}
{\end{itemize}}

\begin{document}

Some text

\begin{notalistitem}
Here is text that should look like a list item although it doesn't
seem to be part of a list.
\end{notalistitem}

\begin{itemize}
\item This is part of a list.
\end{itemize}

\end{document}

Edit: You'll need something more robust if you want to use an itemize inside this new environment, or one of these inside an itemize.