I used to think that also. Then some years back at one of the many coin shows I go to I was telling a dealer that I would like to buy a certain coin of his but would not be willing to pay extra because it was in a slat. He mentioned OK I'll give it to you for an unslabbed price. He bent down below the table and with an assistant opened the slab and showed me the coin unslabbed. I said just as a joke put it back and I'll buy it. He did and I couldn't tell that it had been opened. Later I discussed this with him in detail and he was telling me that many dealers know how to open certain slabs and replace a coin. No big thing, only it is easier with certain ones than others. I always thought all slabs were sealed but obviously not so. Not really sure if this was for real so I asked a few other coin dealers I know. They all agreed. Easier than most people think. Now I look at every slab carefully but just don't know how I would know if it was tampered with.
You're right Carl, there are slabs that can be opened and put right back together. But you cannot do that with an NGC, PCGS, ICG or old ANACS slab. It isn't possible.
Interesting topic. Carl, do you remember what grading company the slab was from? I've heard certain slabs can be popped open with a twist. Also I have heard there is a special tool used to open slabs.