Hi Ghostlight,
Spontaneous mudras (as well as pranayamas, asanas, etc) are an auspicious sign. However, this does not imply the activity of Kundalini. Usually this type of phenomenon relates to pranotthana, which does eventually lead to the awakening of Kundalini (if you proceed with surrender).
These things can be surprising, or even frightening, but they are a natural part of the purification process of Yoga. The meaning of individual mudras may remain a mystery, but rest assured that Prana understands how to proceed. It is working on your behalf and in ways you can't comprehend.
Warmly,
Arthur