Delta Green: Operation FULMINATE

From the web store page:

Operation FULMINATE: “The Sentinels of Twilight”

In which the Agents learn that not all those who are lost should be found.

Since the inception of the national park system, over 1,100 people have been reported missing inside their confines, many of them children. Often, missing children are found far from where they vanished, with no shoes or other clothing. Some are found dead. Some are never found at all.

Something is taking people from national parks, and it might not be precisely human.

This adventure is also contained in the Handler's Guide.