By the time I’ve called the police (which would feel utterly horrible), and the police have confirmed it was just plastic, I’d be a rattled mess for a long time.
(I’m already mildly mentally ill and this would not help)
A similar thing happened where I grew up. An old farm sold to become a strip mall, during preliminary stuff they discovered a bunch of shallow graves. Since they were likely native American given the area (depending on which side of a river it could be native Americans or civil war soldiers), they were reburied in a large mound near the back of the lot.
Since it's next to a pet store, people walk their dogs to piss and shit all over it.
And that about sums up local feelings for natives.
When my ex and I were looking at the house we bought, on our first walkthrough, I noticed the space between two rooms didn't add up, and there must be hidden space behind the paneling in one room. Our agent joked that maybe there's a dead body stashed in there.
Fast forward a year, he writes a short novel about a real estate agent who finds a dead body in a house during a walkthrough.
As it's probably PVC or abs, it can fall apart in a couple of years when exposed to uv. But as it's covered by the deck, it will probably stay intact for a long time.