I am thinking while the locals are busy searching around the bush and the beach, perhaps the Sydney homicide detectives can follow up on every backpacker at Wake-up or nearby hostels, on that day, and may have checked out afterwards and left town.
Looking for a phone, even though it's purple, is still a needle in a haystack job.
Whereas Theo was there at Byron for only a day. How many mates would he have their phone numbers, and WhatsApp with in the middle of the night? Of course if the message recipient(s) was not local, there is nothing to worry about.