[hocuspocus]

In the moment I doubt a top GM would notice. Everyone is preparing using state-of-the-art chess engines and taking inspiration, even from moves that would have been considered crazy a decade ago.

After the fact, it'd probably be more obvious. Human players will typically avoid unnatural moves that require long sequences of perfect play before they pay off.

But top players are also the best qualified at cheating in a way that wouldn't raise suspicion. Many games are lost or drawn because of small mistakes or inaccuracies during the endgame. And playing a perfect endgame is not implausible at this level.