Never far from the Sun, and never in a chart at random.
Mercury never appears more than 28° from the Sun, so it can only ever be in the Sun's sign or one adjacent to it. Given a Sun sign, Mercury's options are already down to three.
It rules two signs in both the traditional and modern schemes, which is unusual — Gemini as gathering and relaying, Virgo as sorting and refining. The same planet in two registers.
Its retrogrades are frequent enough that roughly one chart in five has Mercury moving backwards. That is a common placement, not a rare one, and the ephemeris shows it plainly as a negative daily speed.
What it is
The innermost planet, and the smallest.