Having unexplained assets is not a crime and won't be at least in the US because of the constitution, worst case they could be subject to civil forfeiture. Assuming you did everything else right (full disk encryption, Tor on public wifi/LTE router that you alternate routinely, not using Google, etc) the anonymity of Monero means the government would have a very difficult time tying whatever you bought with the Monero to your actual criminal activity. Of course if you are involved in meatspace crime like drugs then maybe if you use these services they put you under surveillance and eventually find you with drugs in the trunk of your car.