The currency being used has no bearing on whether or not the government will find out if you're buying illegal goods. What matters is the ability of the government to discern the nature of the transaction, which has more to do with its ability to co-opt others (e.g., banks) into identifying and reporting such transactions, and (to a rather limited degree) the ability to prove existence of transactions via the payment method (which Bitcoin is actually bad at since it is basically a public ledger of who paid whom, if you can map names to keys).