In the past (2011) I had a wallet with address: 182TnmtTAStyMyBgAMBrRKNbtC1JqgC6Hp
I bought 1 BTC and spent it. Nothing strange here.
Then I bought 4 BTC and did nothing with it.
Then after some months (in the beginning of 2013) I saw that the 4 BTC were transferred without my knowledge.

Maybe somebody here can explain this?
Are they stolen somehow or transferred to another address within my wallet?
At that time I had a offline wallet (wallet.dat) on my computer which I still posses on a USB stick.

This is the transaction id: 8c11f74362e52c563089125516fabe82907d8469c49d34edc5f8eaaa88eb445a