It's a very large installation, occupying the coast between the San Diego and OC metro areas. There are over 42K service members and 38K dependants living on base: Camp Pendleton | Base Overview & Info | MilitaryINSTALLATIONS."Officials at Camp Pendleton have not confirmed any speculative sightings of Cameron on the installation, but local commands remain dedicated to finding him.
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The family claims a Marine spotted Cameron on Saturday the 31st around 11:30 p.m. and reported active signs of distress before running away toward the ocean."
So the sighting is unconfirmed by officials, but military officials are always tight-lipped. Could Bailey have been on base without there being some record of it? Is it possible from a size/security standpoint that he was hiding on base for an extended period of time?
The terrain is hilly/steep, with various "camps" scattered around within the overall area. So it would be possible IMO for him to be somewhere on base while UA, especially if he was assumed to have disappeared from Carlsbad. The question would be how he re-entered the base. Was his military ID found with his other belongings?