This comes from the old magpie counting song...
If you see magpies, it's an omen of something:

One for sorrow
Two for joy
Three for a girl
Four for a boy
Five for silver
Six for gold
Seven for a secret never to be told

(As used by Magpie, the old ITV series.)

So, if you see a lone magpie, this is a sign of sorrow. By asking "where's your wife?" you imply the other half of the pair, changing it to an omen of joy.
( Amish Information Systems hates misplaced, commas, Mon 19 Dec 2011, 17:16, Reply)