The bin had just been emptied, otherwise I might not have noticed them.
Some of the letters were from the street that I live in, in Bicton, and others were from North Fremantle.
This has happened before, when I found letters behind a hedge in the front garden, and a few months ago, a young man was seen looking in all the letterboxes in the street during the early hours of the morning.
These incidents were reported to the police, and the letters have been posted on to the correct recipients.
Locking letterboxes or at least emptying them each evening would safeguard your mail.