As a 30 + year City employee I am very familiar with the type of person you encountered. After being responsible for or maintaining a specific city property they tend to become too possessive. They feel that it is their property to protect and see you as an interloper. It wouldn't matter if you were a detectorist or simply taking a stroll through the park and stepping on the grass, he/she would raise an issue. As a department manager I have had to call an employee down more than once for that very reason. I have had to explain."Hey, Bob or Jane, it's not your personal property and that person has a right to be there, so back off". What I would do in that situation is find out who that person's suppervisor or division head is and go to them, armed with the ordinance, a copy of the detectorists code of ethics and any other information that sheds a good light on the hobby. I would lay it out just exactly why my detecting is not harming the property and even demonstrate digging and replacing a plug if the person was interested. What I find is that most people thus approached are very receptive and interested in metal detecting.
Another scenario is the slob detectorist who leaves open holes, digs twice the size plug that is necessary and instead of picking up trash as they go, just tosses undesirable targets aside and leaves them for someone else to pick up. I have seen that person too or seen where they have been. It give a black eye to all of us who enjoy MDing. Don't be a slob and if necessary pick up after a person who is. If you see him or her being a slob, don't be afraid to remind them to pick up after themselves. Show them how to remove and replace a plug with minimal damage if necessary. Be diplomatic in dealing with all persons who might affect our hobby and don't be like the guy who jumped you out. Just some advice from someone who has viewed the issue from both sides. JIM