to emodendroket's point, I recall reading an article that in the previous year (2016) that the IRS stated that very few people were claiming bitcoins in their taxes. I'm not versed on cryptocurrencies so I don't know how plausible it is that they know this, but it seems like something they're looking at now.
Basically, only 802 people told the IRS about their 2016 profits in CoinBase, one of the more popular exchanges. It's safe to assume that number was a tiny fraction of the actual US users.
I imagine it is like anything else: they don't know, but if they do an audit or another agency is investigating you for something else and they notice, then they start investigating it.
I do think it is fair to say that there are more than 802, apparently, people in the United States in 2016 that mined/bought/sold bitcoin so they definitely see there is a problem.
