Yes the problem is detection and enforcement and not clear how to achieve this. It is imperative that we someday overcome this though as the risk of accidental escalation at any point in time is significant and is bound to happen some day. Everyone must come to the recognition that possession of nuclear weapons is an aggression against everyone given their widespread impact and it is in everyone's self interest to not allow for their existence. I'm also not sure how easy it would be to keep large quantities of weapons completely hidden. There is also the issue of incorrect or false accusations of possession of the weapons to allow for aggression against an entity, it hasn't even been very long since the last time this happened either.