The US is one of few countries where dual citizens must pay dual taxes regardless of their primary residence location.  That is to say that US citizens must pay the same US taxes on their income regardless of where it came from and/or where it resides.  If there is an exception to that, it would likely be one where taxes where higher when money/income were not in the US.  They also have plenty of other ways to track your income, for instance, any US-based exchange and likely any exchange in any in any of the fourteen eyes countries.  Even trading outside of that jurisdiction doesn't guarantee they won't know, for instance, see the btc-e.com takeover landing page, where the IRS logo is present amongst other US LEA logos.