Blizzard tracks all programs your computer runs (check the ToS if you’re skeptical, it flat out says they can and they do), and stores this information for some time. Contrary to some other posts – although they do watch for processes attaching to the game, they also simply scan everything the computer runs.
Last two days isn’t really relevant. They might have detected something they didn’t like months ago.
It doesn’t really matter if you did or didn’t. Blizzard’s detection software must’ve just picked up on something. If you didn’t use Visor or Pursuit, just ignore the email and go on with your business.
OverTrack is basically a similar program except it doesn’t offer any advice or tips, it just monitors your in-game stats and what happened during the match (e.g. when the payload was being pushed or such).
It gives you information Blizzard doesn’t want you to have.
Personally I think it’s a bit asinine. How long until someone makes a version of Pursuit that pulls directly from Twitch VODs, never runs on your computer and thus is completely undetectable?
But it’s Blizzard’s game. They say running Pursuit is bannable, and thus it is.