The Vanishing by Jayne Ann Krentz

Decades ago in the small town of Fogg Lake, The Incident occurred: an explosion in the cave system that released unknown gases. The residents slept for two days. When they woke up they discovered that things had changed—they had changed. Some started having visions. Others heard ominous voices. And then, scientists from a mysterious government agency arrived. Determined not to become research subjects of strange experiments, the residents of Fogg Lake blamed their “hallucinations” on food poisoning, and the story worked. But now it has become apparent that the eerie effects of The Incident are showing up in the descendants of Fogg Lake.…

Catalina Lark and Olivia LeClair, best friends and co-owners of an investigation firm in Seattle, use what they call their “other sight” to help solve cases. When Olivia suddenly vanishes one night, Cat frantically begins the search for her friend. No one takes the disappearance seriously except Slater Arganbright, an agent from a shadowy organization known only as the Foundation, who shows up at her firm with a cryptic warning.

A ruthless killer is hunting the only witnesses to a murder that occurred in the Fogg Lake caves fifteen years ago—Catalina and Olivia. And someone intends to make both women vanish.