By the time Iāve called the police (which would feel utterly horrible), and the police have confirmed it was just plastic, Iād be a rattled mess for a long time. (Iām already mildly mentally ill and this would not help)
I'd do my very best to see if it was realor plastic before I called the cops, they would probably try to blame you for the murder.
If you find something that looks like human remains hidden in a shallow grave, you really shouldn't go poking around at it and disturbing it. If it is real, the forensic people will be pissed if you've been down there giving it a good fondle before they get there.
I'm less worried about the forensic people at my local PD (They don't have one) and more worried about the very real possibility that no matter what evidence is found, it will get pinned on who found it, because cops in small town Okkahoma are not the same as cops on TV.
EDIT: Also, it's worth mentioning that anyone who has a dog will be able to determine if that's a plastic skeleton or not.
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.
Still can't get over that.
Plot twist: Someone took this opportunity to get rid of a real skeleton by selling it anonymously as a plastic one.