Yeah, this is always a tricky/sticky situation. If you did indeed flash the card (even if only you saw it), it should be removed from play and re-dealt (and become the burn card). But if you merely *think* you saw the card, it can cause unnecessary hardship if you are incorrect.
It's certainly not fair to say nothing (giving you a potential advantage), but it's also not fair to the affected player to make him expose the card to the table if you are wrong. It's also not fair to announce to the players what you *think* it is, because if wrong, they'll also know that the player *doesn't* have that card in his hand. Either way, everybody else has info regarding that player's hand that they should not have, due to no error by that player.
The best approach is to send a face-down note to the player with the card rank/suit in question. If he confirms that it is correct, he surrenders the card as an exposed card (and is re-dealt a new replacement in accordance to procedures). However, if he denies that it is his card, then you as the dealer have a dilemma -- you can either announce the non-card to everybody (unfair to the player in question), or keep the knowledge to yourself and remove yourself from the hand (unfair to you, but hey, it was your mistake). I favor the latter approach.
This goes for other players who occasionally think they see flashed cards, but are wrong. They can send a private message to the player, but if wrong, are disqualified from playing that hand. It's the fairest way to deal with the issue.