The thing happens in a mysterious way, when synchronizing the wallet after the years, the transactions disappear....
That will happen if sometime during those years the bitcoin was send to another adress.
In such case, Bitcoin Core would show additional transaction where the Bitcoin is moved rather than delete existing transaction from transaction history.
[1] allprivatekeys[dot]com/wallet.dat?mined=on
That is true, i assumed he meant that the bitcoins disappeared from his wallet balance. Because there is no way that the transactions would disapper from his wallet, even if it got hacked or something of that kind. Actually the OP should just take his addresses and check them on an block explorer. Its not that hard and he would know in a few seconds if he still has the bitcoins or if they were sent away, stolen or whatever.