Probably they were using a database, which is entirely legal in daily games.
What constitutes "suspicious" behaviour?
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Wouldn't worry about it. Opening ends when you deviate from book. If you play 4 moves and the 5th deviates, it is not hard to have a high opening and confusing middle game. Especially if the line you deviate at is something the person can't remember or more likely hasnt looked at, as it is just dubious, but they dont know the only line, or just blunder.
I finished a daily game earlier and the game review had my opponents accuracy level in the opening at 99.6% but their middle game was 29.1%
Is this just a case of someone who's memorised an opening that they play every time but as soon as they have to go off script, it all falls apart?
The player was a 400 ELO player so I'm not sure if that makes things more or less plausible?