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The public keys of each and every one of the earliest mined bitcoins are viewable publicly on the blockchain. These keys were created using ECDSA encryption. ECDSA is perfectly fine and pretty much unbreakable with even supercomputers, but it can be cracked with a quantum computer using Shor's algorithm.

Now, almost every wallet is safe from this sort of attack. Only very, very early wallets have this vulnerability. Wallets like Satoshi's wallet, with 1.1 million BTC currently valued at ~$113,853,410,000. Of course this would be easily preventable if Satoshi were still around. Governments, banks, and other institutions will be transitioning to quantum-resistant cryptographic methods over the coming years (they already are). But Satoshi, and many others, can't do that. Those keys are lost and gone, those wallets are set in stone.

In order to compromise those wallets, you'd need a quantum computer far exceeding what we have today. But part of that is the high inaccuracy of quantum computing which necessitates many physical qubits per logical qubit. This is a problem that's already being worked on (big news just 2 days ago), and larger quantum computers are already being developed.
IBM is already planning on creating a 1386 qubit system this year. It's only a matter of time before quantum computers reach that magic number and those wallets are breached. Yes, the scientists working on quantum computers already know all of this.

You will have no warning, the first sign will be those coins moving.
The best you can hope for is Bitcoin forking to a new, quantum resistant variant before this happens. Hope that doesn't crash the price lol.