I found the following in Section V of the Rules and regulations of the Lake County Forest Preserve website. While it doesn't say the verbiage specifically "Metal Detecing", it does outline what it is that we are basically doing which is looking for misplaced items...
SECTION V
USES PROHIBITED
A. Protection of District Property. No person shall commit any of the following acts on District Property:
(i) destroy, deface, paint, alter, hide, change, break, mark upon, damage, take, remove, or excavate any District Property;
(ii) tamper with, enter, climb upon, damage, take, or remove anything from any District Personal Property;
(iii) deposit into any refuse container any garbage, trash, refuse, or other material that was not generated in the course of normal, lawful use of District Property;
(iv) deposit, dump, or place any ashes, trash, rubbish, paper, garbage, refuse, debris or junk, except in containers provided by the District for such purpose;
(v) climb, stand, sit or walk on any District Property not designed or intended for such use; or
(vi) collect, gather, acquire or scavenge for lost, misplaced, or abandoned personal property, including without limitation, golf clubs, balls, clothing, other than personal property owned by such person.
B