So you're a DM and there's this thing that your players are unaware that still calls for a saving throw. Let's say someone poisoned their mugs of ale with a sleeping potion that won't take effect right away. You ask them to roll for Poison without explaining more. Or you roll a die yourself while hiding the result (the 'good' ol' DM secret die roll!) and look at their character sheet. Or if you're sneaky enough, you roll a die yourself and look up the saving throw value discreetly/have it written somewhere/remember it. WHATEVER! To quote Rocky Balboa, "Let me tell you something you already know!" (+100 xp if you read it with Stallone's voice in your head); The players now suspect something and will behave differently, even if they genuinely try not to meta-game.
Oh what's that, you sometimes roll for nothing so they won't know when you're rolling for real while grinning so they'll be scared or something? That gets annoying really quickly. "Oh look, he's rolling AGAIN while grinning, same as he did five minutes ago." And if instead you don't roll them often but sometimes you roll to bluff so they won't know when it's real, they'll still change their behavior anyway because it doesn't happen so often that they'll ignore it!
So what's a DM to do? I dunno for you, and maybe you'll even tell me you like the methods I've just dissed and that's fine if you do, but this is what I did the other day: I took a sheet of paper, asked every player to roll their d20 (for saving throws) a few times while I wrote down the result of each throw. Then I asked them to do the same with a d6 since the game I'm running uses them for skill checks. What does that accomplish?
Well, I take notes often. If players see me putting pen to paper, they don't know what I'm checking. They don't know I might be scratching off one of the dice results. If you think even that might give a hint, then simply remember where you
are in the list of pre-thrown dice rolls and scratch off the used ones
later after the game or when no one is looking.
Players will finally be none the wiser.