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)
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.
