hi john, congrats on getting permission to hunt some old sidewalk real estate. they can be very productive. i think you'll do well, and end up with a lot of sidewalk to do. despite the large number of posts and responses, there seems to be some unanswered questions regarding the rights of us detectorists to hunt a combination of public and private property - it can be done - the same way you did it.
i have read most of the answers to your post, and feel compelled to answer. in my nearly thirty years of detecting, i have always found it to be much to my advantage to ask permission of the owners, whether they are private, county, city, etc. some cities own all of the sidewalk and a couple of feet on each side of it; some cities own only the sidewalk and the road frontage; and some cities own only the sidewalk itself. it then becomes our responsibility to find out the true owners and secure permission to hunt. one trip to city hall will settle this question. ignorance of the law is no excuse. i commend you sir, for taking the time to find out for yourself.
let us not be negative and disagreeable on this subject. it looks pretty clear cut and simple to me...property rights should be respected, and we should ask permission to detect, no matter where we are. nothing less will do. we have a good image to project and protect to the non - knowers. i do wish you luck on your quest, john, and hh,