Re the Pct 4 parks. Are there any signs up saying metal detecting is prohibited? If there are no signs I really dont think the constables can arrest you! For an activity to be 'illegal' to result in an arrest, there has to be a law or rule! I have researched the Harris County Commissioners courts rulings and I could not find any rules about metal detecting except for the sites near Jesse Jones park near Humble. If no sign, there is no law or rule.
But, if you want to metal detect, try Old Town Spring. The parking lots to the south are pretty good. I have found lots of clad and even an 1854 dime. An old general store used to stand at the nw corner of Bammel Road and Hardy. Was moved in the early 70's. There are lots of areas in the downtown area that are under construction. If no fence is up try it. Found a 1799 Mex 1/2 Reale (bout the size of a dime) at the old Bland Cadillac place about 15 yrs ago. And the beachs are always good for clad, and sometimes a ring or two.