The Night Searchers by Marcia Muller

PI Sharon McCone gets another convoluted case in The Night Searchers, involving a man whose wife has supposedly witnessed some bizarre goings-on. Sharon thinks she might need a shrink more than a PI but agrees to investigate. That investigation runs into a case Sharon's husband's security firm is working on, a kidnapping. The next thing you know, Sharon is fishing a man out of the bay--and finding herself under the scrutiny of Homeland Security. And everything seems to revolve around a mysterious group called The Night Searchers.

It's a good story and a quick read if you're in the mood for a mystery on a hot summer weekend.