So you want me to be serious. Sure you're feeling OK?
The edges and corners are very sharp. Either a building came down or something, maybe that plywood, had laid there for a long time. I don't know what kind of grass you have there but I'd suspect a small building and not plywood just laying there as I don't see grass runners going across it like they would under plywood. At least they would if it were Bermuda or some other grasses. That would be a mess of runners. Besides, the sized don't look right for the plywood to have caused it.
That is not a freshly dug hole. It's too small and neat to be machine dug and I don't see the debris or traffic marks I'd expect with either machine or hand digging as you pointed out. I guess it's possible you filled a hole and hauled in the fill in a wheelbarrow running over that plywood to avoid soft ground. If you did you are neater than I would have been. But that looks pretty dry and you've been commenting about wet weather. So maybe you did haul in fill. But where would you get dry fill?
I don't get your reference to the Federal government. You said it was our government. That can only be the Feds. Anything state or local is on you buddy, not me.