I learned the hard way that racoons will also come along at night and re-dig holes that I dug and filled in. After detecting my parents property I got a call from my Father the next morning telling me to get over there and fill in all those holes I left in the back yard. I most certainly filled them in the day before but when I got over there sure enough the back yard looked like a war zone.
It took me awhile to figure out what had happen but after setting a live trap and catching and relocating racoons for the next week the problem was solved. I'm not saying thats what happened there but I know it does happen. As far as punks getting parks declared off limits to detecting...it happen here. Some bozo was using a folding army shovel to dig basketball sized holes and killing the grass. Some people just don't get it.
Hey now, I'm that bozo sometimes. I pinpoint as best as I can, but I can get it down to about a 8x8 inch patch at best. Especially with this dry spell we've been having, it wreaks havoc on detecting for some reason. I cut my plug with a shovel, flip it over, detect, find the item, and then put the dirt back in the hole with the grass on top. Its not perfect, but its pretty good. Unfortunately with this dry spell, the grass will probably die (its mostly brown already).
Its just how it is. You won't be able to cut a plug around here with one of those handheld serrated jobbies. The clay and rocks and hard soil just won't let you. Heck, its pretty darn hard even using a shovel right now to dig a hole.