That happened to me exactly the same way, back in December. Finally, traced it to Bittrex. Lessons learned: Hacking an individual's machine is easier than hacking a bank. Top security practices are needed, else, keep your money in the bank.
Yes lesson learned.
I found out that POS coins like ARCH are a greater security risk if you are staking them as you make yourself a target for people prepared to hack as anyone with a wallet can see your on the network 24/7