It went to the change address.
All the bitcoins in an address are moved when a transaction is done, the unspent bitcoins just are sent back to the same address or, in case of the bitcoin-qt client, to a new address, for anonimity and security reasons
The bold part is not entirely accurate. All the bitcoins from a previous transaction are moved when a new transaction is done, but if an address received bitcoins from multiple transactions, you may find that only some of those previous transactions are spent, while bitcoins from other previous transactions to that address remain at that address.
Example:
An address receives 3 transactions, one for 2 BTC, one for 5 BTC, and one for 9 BTC.
This is the only address in this wallet, and it has never received any other transactions.
Then a transaction is created with this wallet to send 6 BTC to a new address. The wallet could choose to spend the 2 BTC transaction output and the 5 BTC transaction output for a total of 7 BTC, sending 6 of those BTC to the recipient and 1 BTC back into the wallet at a new hidden "change" address. Meanwhile the 9 BTC output wild remain associated with the original bitcoin address where it was received.