What prefix to imperceptibly?

There is no prefix that can be added to "imperceptibly" to form a new word in standard English.

"Imperceptibly" is already a complete word meaning "in a way that cannot be perceived."

However, if you're looking for a word that conveys a similar meaning but with a stronger emphasis on the lack of awareness, you could consider using phrases like:

* Unnoticeably

* Subtly

* Insensibly

* Covertly

* Stealthily

* Unconsciously

These options convey a similar sense of something happening without being noticed or perceived.