As a long time coin collector and dealer, I have bought and sold many mint and proof sets that fall under the "unopened" catagory.  The flat packs from 1955 to 1964 tend be be unopened if they are from boxes shipped from the mint that have been stored in humid environments.  They are true unopened sets if the original box remained unopened from the mint or in the case of a single set, if the shipping envelope from the mint with the delivery stamp and address is uncompromised.  When buying unopened proof sets, look for "sealed mint box" or "unopened mint box".  These are proof sets in the original mailer from the mint.

Many of these unopened sets are spotted and toned due to the moisture influence.  The integrity of the envelope is most important.  If the envelope appears to have water spots on the back, chances are the envelope was previously opened with steam and resealed.  In my experience with unopened sets, cameos are not uncommon.  Deep cameos are extremely rare and the corresponding price for a high graded specimen justifies this.  Do not expect to find deep cameos with any frequency in unopened sets.  You have a better chance of finding quality cameos or an error coin in those specific dates where they are found.  Either way, purchasing an unopened set constitutes a treasure hunt or a gamble and being from Las Vegas, I enjoy a little chance.  Besides, the odds may be in your favor of finding something when compared to other games of chance at the casino.  Please register your vote if you found this guide helpful.  May Lady Luck smile on you!