I also had an unauthorized transaction yesterday, I've been using blockchain for about 2 years now and have never had this problem before. I talked to support and they said i either had malware or used a phishing site. I'm using Ubuntu so I don't think malware is the problem, I was in a hurry when i logged in but i tried to look for phishing sites by doing a search and couldn't really find any( i also just made this wallet). The only other thing i did was use tor while creating this wallet(for anonymity) not sure if thats what did it, but I've done this multiple times before with no problem. Guess this is just the nature of the btc game, i should have set a secondary pass maybe? any insight would help for future transaction. this is the wallet for reference 1CDAeLd55L4o9ivYGHsH2JecB3HfRgVKYt
Thera was phishing site yesterday on top of the google ads (so in front of the normal search results) for a few hours, when looking for blockchain.info on google. Google removed it eventually.
The site looked exactly like the normal blockchain wallet login, but the url was something different like "blockchain.info.xxxx.xxx". I have two factor autentification and the field for two factor authentification did not show up. I did not enter anything on this mask.
Maybe you went on this phishing site and you did not have two factor authentification on. Then the phishing site stole your password and they emptied your wallet.
My real account on blockchain.info is just fine, nothing missing...