"Island" by Gordon Koleman (2nd book of 3 series trilogy)

The two-minute drill would best be described as an outdoor lockdown drill. This drill includes tearing down down the canopy, putting out the fire, hiding the raft, and hiding themselves.... security drill.

The signal came from Charla, atop a palm tree — the hooting of an owl, a sound that would never be heard on a tropical island. That set the vanishing process in motion. The fires were extinguished, the stills folded up and buried in the sand. A few sweeps of a giant fern and their footprints were gone too, leaving a deserted beach.

Two quick kicks took care of the supports for the sun canopy, and the lifeboat lay flat. Ready hands drew a leafy blanket of woven vines and palm fronds over it. Suddenly, the black rubber craft was gone, replaced by the green-brown colors of the jungle. Finally, the castaways themselves disappeared, melting into the dense underbrush.