Call 911, tell them where it is, explain that you found a trash bag somewhere and you're concerned it has a dead body in it, don't disturb it any more than you already have

I work in 911 dispatch, from my end of things this is a very straightforward call. Verify your location, one or two short lines of notes, send a cop out to check it out.

I've taken a few calls like this, luckily it's always just been trash or at worst a dead animal.

One time the responding officer found some bones in the bag and was pretty sure they weren't human, but called out our on-call coroner to be sure who confirmed that it was just a deer or something.

Similarly I once had a call from an off-duty coroner reporting a "strong smell of decomp" from the woods near a gas station or something. I guess if anyone would know it would be them. Sent a cop out, sure enough, it was a dead deer.

It's very rare that anything like this is ever as exciting as your imagination makes you think it might be. Still, always better to call if you're unsure.