The Australian-English phrase Malley’s cow is used of someone who has departed and left no indication of their present whereabouts.

In A Dictionary of Catch Phrases British and American, from the Sixteenth Century to the Present Day (Taylor & Francis, 2005), Paul Beale wondered, of Malley’s cow:

could this be the mate of the beast to be found at fit as a Malley bull . . . .

The earliest occurrences of this phrase Malley’s cow that I have found are from The Territory (Sydney: Angus and Robertson, 1951), an account of Northern Australia and its people by the Australian journalist and author Ernestine Hill (1899-1972):

– Ernestine Hill used the phrase on several occasions in her book—for example in the following passage:

When the manager came in with the mustering camp and heard of that old city josser under the green umbrella making off into the desert where nothing lives.
“He’s Malley’s Cow,” he said. “He’s a goner. We’d better go out and get him, or we’ll have to go out and bury him.”

– In the glossary appended to The Territory, Ernestine Hill defined Malley’s Cow as “A person gone away”, and explained:

Malley’s Cow is the Australian equivalent of Alfred and the Cakes ¹. Back in Monaro ² folklore one Malley in a mustering-camp was told to hold a particular cow. When the boss came back and asked for it, Malley grinned. “She’s a goner!” he said. Hence the proverbial Malley’s Cow.

¹ This is the story of King Alfred and the Cakes, from First Steps in Reading and Learning on the System of Word-building (London: Longmans, Green, and Co., 1867), by the Canadian educational writer Alexander Kennedy Isbister (1822-1883):

One of the best of our old kings was Alfred, called the Great. When he came to the throne, he found the land full of strife and blood, for a fierce set of men, called Danes, had come and made war on the land. He fought to try to get back his land from them, but they beat him, and he was forced to flee to save his life. He found his way to a small isle, a place you could not get to but by a boat. Here he met a poor man, who took care of swine, and went home with him to his house. The man’s wife brought him some food; but she did not know who Alfred was, she thought he was some young man who had been to the wars.
One day the good wife set the king to watch some cakes which were put to bake by the fire. But as he sat he thought of how he should fight the Danes, and did not think of the cakes on the hearth at all. When she came back, she found them burnt. She took them up in a rage, and said, ‘You man! you will not turn the cakes when you see them burn, but you are very glad to eat them.’ She would not have said this if she had known he was the king. But while she spake he rose up and told her who he was. The man and his wife did not know what to do or say, till the king took them by the hand, and bade them not to fear; and when he got back his land from the Danes, he was kind to them both.
When King Alfred by the Danes was beat,
In a swine-herd’s cot he took re-treat;
One day, while sitting thinking by the fire,
He burnt the cakes, and roused the housewife’s ire

² Monaro is a region in the south of New South Wales.