Small drip could mean that the poppet..the small piece that goes up and down....is cracked. If it is leaking from the threaded area of the bonnet....it could be a hairline crack in that area as well.
Sometimes the freeze cracks them, sometimes not.....the drip will only get worse..and will irritate you no doubt, replace it if you feel the need.
Examine the brass body carefully for damage and/or nicks in the threaded area....that could also be a problem.
I have several folks that I've worked with in the Phoenix area, if it turns out you need a contractor, contact me.
Good luck....
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net