Be careful with this, remember the Feds took down the mob by proving tax evasion, not actual crimes.

If the US government (with all of the data collected by NSA, etc) is able to prove that at one point you acquired a certain number of coins then it is easy for them to force to you admit to one of the following, either:
1) You sold the coins, in which case you better have reported that past sale to the IRS and paid taxes, or
2) You still control the coins