If someone gets access to your private keys, they can create a transaction that spends your coins. This transaction can be created and sent from any computer in the world. There's no need for your computer to be involved, and it's irrelevant whether your wallet software is open or not.
It's possible that some person or malicious software had access to your computer at some point during the last year (or before) and copied the file which contains your private keys. If it was encrypted, they would need to decrypt it, but if your passphrase is simple enough, that may not be very difficult. Their software may also have been running earlier and recorded your passphrase at some point when you typed it, or extracted the private keys from the computer's memory while the wallet software was running. For whatever reason, they may have waited some time before spending them.
If there were other copies of your private keys somewhere else (such as in a backup, or on an old computer or phone that you gave away or had stolen), then that is another place they could have got them. In that case your computer needn't have ever been involved.
But you should probably assume at this point that your computer is compromised. The safest course of action is to back up all your data files (but not the operating system or application programs), wipe the system and reinstall it.
