If a mixing service is used it is practically impossible to "follow the coins" via the blockchain.

Consider a payment of 5BTC is made from user X to user Y via a mixer.

User X pays an intermediary, or mixing service, which takes that 5BTC and splits it up into a variable number of finite amounts adding up to 5BTC.

The mixers first wallet then sends out each of those pieces to other mixer wallets it maintains, which then do the same thing as that first wallet.

Ultimately, a certain number of wallets maintained by the mixer combine to send the end-user's wallet the 5BTC total (Usually minus a mixing fee).

I've seen this brought up a number of times on the forum as well as /r/bitcoin, and while it's theoretically possible I doubt the Fed even knows what Bitcoin is at the moment. It's still a fringe currency. While the scenario you've set forth might theoretically work, I imagine a $1 billion market cap currency is too small to consider a threat from their perspective.