Not sure after reading LoyceV's response but my basic instinct is saying that you should be safe. I do not see any reason for a client to start broadcasting a tx that was made long time ago and never was broadcasted.

Do you know the private key of the wallet? Assuming you do. In that case try to restore the wallet using Electrum and once you see the balance then send them to another wallet (fees are too high right now which sucks by the way). So once the address is empty (make sure the tx has minimum 1 confirmation), then try to restore the wallet using your core client. This is the safest practice I can think off now.

PS: Make sure you have verified the signature of the downloaded Electrum client. Here is a tutorial if needed : https://bitcoinelectrum.com/how-to-verify-your-electrum-download/

Edit:
The reason is that the client has not synchronized in full yet.

PS: I am assuming that you have not sent the coins to a wrong address since you said the coins were there (core).