Someone knocking on your door and asking how you are and about a missing item is not trespass. Entering a premises unlawfully IS, or staying on a property after being asked to leave IS.
Per Australian High court ruling:
"The occupier of any dwelling-house gives implied licence to any member of the public coming on his lawful business to come through the gate, up the steps, and knock on the door of the house. This implied licence extends to the driveway of a dwelling-house. However, the licence may be withdrawn by giving notice of its withdrawal. A person who enters or remains on property after the withdrawal of the licence is a trespasser"
WA state law goes on to say that after asking someone to leave, and they fail to do so, REASONABLE and EQUAL force can be used to remove them.
But that is drifting off topic.
OK, so he fails to answer phones, emails, faxes, courier pigeons. No one seemingly has seen him. Easiest first action is for someone to visit him to see if all is OK, and politelly and truthfully explain to him the situation.
If that fails, or if he cannot be found, an enquiry to WA Police may be in order.
No one is talking about smashing down doors with baseball bats and bolt cutters. Geez, don't go jumping to conclusions about the word posse
Just means sending a search party. It may be the West, but it's not the wild west (well, until you get to Bindi Bindi or Narrogin)