It's unlikely that prospecting goes on any more anywhere in the United States; the population is so dense that almost all natural resources that exist here have been discovered and exploited.
I would say yes there are. People look for gold all over the world. If this is a place where gold has known to be found you will get someone prospecting if only for a hobby. Some do it for their holiday or as a weekend pastime, it can be a fun thing to do.
This question seems to be off-base. Sonoma isn't in the gold country. A question that would make more sense is "Are there still people gold prospecting in Sonora, CA? And the answer would be yes, there are people prospecting in the Sonora area, if not in Sonora proper.