[5e] Dominate Player - How do you handle a player being dominated?

I'd like to hear how folks handle when a player has been Dominated?

Specific to my situation, I've cobbled together what I guess would be table etiquette for my group that has served well so far, but I'm afraid it could lead to trouble for their first Mindflayer encounter:

Secret Rolls - I make secret rolls for certain situations, relevant to this would be the wisdom throw from Dominate Monster cast psionically by a mindflayer. Since there are no components to cast, there would be no visible sign, therefore the player's wisdom save will be made in secret behind the screen. Should a character (not the player) know they've been dominated or in some way affected? Or does the magic twist their perception enough that it all feels 'normal' until the spell ends and they realize what they hath wrought?

Open Table - I don't pass notes or take anyone aside and instead trust them not to metagame, to prevent weird player secrets and also to enhance the overall storytelling nature of my campaign. They've done well so far but it also means a deep secret - such as being compelled to behave differently by a mindflayer - will get spoiled in such a setting no matter how good they are at avoiding metagaming. I'm considering handling this by telling the player what their character does without further explanation, such as "Flint decides this conversation isn't worth it and turns around to leave" or "Mildred turns to stab Flint, roll for attack", and staring at them deadpan when they ask why.