I do agree with that statement,as I am a Military Police officer.But I have good relations with the foresters from the region.
The law on MD is about the same as in france.
On public domain you can't do anny MD diging without the autorisation from the provincional governor(almost impossible to obtain in serten provinces),anny find must be declared to the propper authority( if not you wil be proscecuted for theft from the state,reward is about 10% of the objects worth)
If human remains are found you must inform the police.(privat or public domain)
Explosives and by that I mean even a bullet,cannot be retrieved,as of then you fall under the weapons act(In belgium you have to warn the military bomb and UXO squad: DOVO)(It's illegal to posses anny ammo without the proper liscense)
Weapons fall under the same act even in non working state(you need a collectors liscense)
On privat domain annything go's aslong that you are in order with the law and the owner of the domain.
For example A large portion of WW1 trenches in Belgium are on privat domains,and the farmers have no problem with it(always ask a written permission from the owner)But keep in mind that this region got pepperd with all kinds of gas shell's and there are a lot of them left.