he probably spawned but you never saw him .. hes a sneaky bugger and will spawn ofshore but high in the air and with his stealth you have to be pretty close to spot him so you defo need flyers to scout around the island.
Defiant's GoEH/MILITIS METUS
when he 1st came out we spawned 4 not knowing that we did, as what was said you have to keep an eye out in the air just keep going around the island killing until you spot him, maybe you will get more than one 
i had a team going around kill the spec, but we still did get caleb to apper anyone know what we did wrong, we done it to dusk to dusk and dawn to dawn, but he still didn't show. and there seem to be little info on how to get him