How to remove top margin in Latex Beamer

I'm playing around with Beamer themes and currently designing a custom title page. However, I can not figure out where the margin between the top of the frame and the top of the text comes from.

MWE:

\documentclass{beamer}
\begin{document}

\begin{frame}[t,plain]
Test
\end{frame}
\end{document}


There is additional space between Test and the top of the frame. Where does it come from?

The \beamer@frametopskip is normally set to .2cm plus .5\paperheight for top aligned frames. You can change it like this:

\documentclass{beamer}

\makeatletter
\define@key{beamerframe}{t}[true]{% top
\beamer@frametopskip=0cm plus .5\paperheight\relax%
\beamer@framebottomskip=0pt plus 1fill\relax%
\beamer@frametopskipautobreak=\beamer@frametopskip\relax%
\beamer@framebottomskipautobreak=\beamer@framebottomskip\relax%
}
\makeatother

\begin{document}

\begin{frame}[t,plain]
Test
\end{frame}
\end{document}


(using a different background colour for the image so one can see more clearly where the frame stops)