The lost letter

The letter was written on 10 April in the UK.The date stamp suggests it was processed in Australia on 4 August. It arrived in our mail box today, 9 August.
The intriguing question is, where has it been for those four months? Did it lie overlooked in the bottom of a mail bag (or whatever they put mail in nowadays)? Or has it been on a joy ride around the world? Or did the sender forget to post it? We will never know.