finding body parts on the lawn

So I started the day like most others. Blaring alarm, sticky eyes, slight headache. Grab my robe and let the dog out. Sit out on the porch for a few minutes smoking a cigarette, watching the dog run from side to side.
Back inside to check the email and hop in the shower... wondering how anyone showers before they go to bed. Doesn't matter. 10 minutes, in and out. Put on the same clothes from last Thursday and head for the door.
Crap, the dog. Grab some food and water and put the kid in her pin for the day. Lucky bitch. All day to just bum around and do nothing. G-d she must get bored.
Grab my lunch and head out into the garage. Gotta take the garbage to the curb... damn this thing stinks. Halfway out, I see it.
What the hell is that thing?
It's not every day I see a random arm on my lawn. An arm? Where would an arm come from? Is it from some christmas menagerie from down the street? wha.. is it real? Holy shit! that is a fucking real arm! Damnit! I don't want to clean up someone's arm. The trash can is close...
No unusual cars on the street, too early for anyone to be out... I grab a trash bag and toss it in the can. Put the can out at the street and go on with my day. Why didn't I call the police? I don't really know. I just didn't want to deal with it.
Nothing happened for the rest of the day... at least nothing worth mentioning (except for the earlier post about shooting me, but I've since explained that). I get home at dusk, and notice a note on the door. Figured it was from FedEx or UPS or something. I don't exactly rush to go pick it up. Thought the plants needed some water, got the mail, took the trash can in (empty, no issue). Finally made it to the door to get the note... not UPS or FedEx, just a small piece of white paper taped to the front door. Something written inside.
"Witness"
The crappy Harrison Ford/Kelly McGillis Amish movie? I don't think so. Someone saw something. I don't really know what to do about that. I mean, I just found the arm, I didn't put it there. What can they really do? Or, do they assume I was a witness to something else?
I guess I'll find out over the next few days.