Disclaimer: I have no direct knowledge of this issue. I am passing it along to encourage Mac/Time Machine users to exercise caution.
This person experienced data loss on his Time Machine backup drive (of which he did not have a backup) after installing Sophos Free AV for Mac:
http://recoveringphysicist.com/17/did-sophos-free-a-v-for-mac-kill-my-time-machine-backupsPatrick
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