Given food, moisture, and a dark space, mold can thrive in a variety of spaces. Unfortunately, that includes spaces that are hidden from your day-to-day view, like behind your wallpaper.
If you notice peeling edges of your wallpaper, in a moisture-heavy room or smell a moldy odor, you may have hidden mold. Lift a corner, near the floorboard or other out-of-sight areas to look behind the wallpaper. Keep in mind that removing the wallpaper can lead to a large release of spores, so before removing all of it, consider hiring a remediation company.