In Avengers: Endgame right at the end,
Iron Man steals all the Infinity Stones right from Thanos.
someone gets the stones,
it's this huge deal with electricity, etc. So,
why didn't Thanos notice the Infinity Stones were gone?
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Iron Man uses his nanotech to steal the stones, and if you watch carefully when Tony Stark's hand comes around to reveal he has the stones, the nanotech in his suit is still forming the gauntlet itself and moving the stones into the right positions.
Once they are located in the correct spots, the electricity starts to spark as you expected and Tony wastes no time in getting the snap done.
Thanos didn't notice the stones being taken off the existing nanotech gauntlet because Tony only grabbed his hand for a second or so and didn't get the gauntlet off him - it didn't occur to him that there was any way in which the stones could be removed from the gauntlet in those few seconds.
Dave's answer covers why Thanos didn't notice Tony sparking. But it doesn't explain why he missed the lack of pain on his own hand.
But this Thanos is from 2014, and had never seen the fully complete Gauntlet before. He didn't know to expect excruciating pain upon activating it.
Also, Tony's Gauntlet works differently from the original Infinity Gauntlet. If you watch Thanos add the last stone in Infinity War, the Gauntlet surges in power, but then the surge dies away. Even if Thanos was familiar with the specs of the original Gauntlet, he would expect the power surge from the Gauntlet to stop, and he wouldn't know that Tony's weaker Gauntlet never stops sparking and surging. So when the pain vanished while he was fighting with Tony, it didn't occur to him that something was wrong.
I know I'm too late for this question, but it didn't occur to me until now, why Thanos could have never suspected Tony stealing the Infinity stones from him; it's because of his arrogance in the Final battle. He himself says that to Nebula soon after arriving to the Future.
Nebula: "They didn't suspect a thing"
Thanos: "The arrogant never do"
Obviously Thanos became arrogant from the time he saw so many Avengers join Captain for the battle; he rains fire on his own troops. He's devastated to see Cpt. Marvel destroy his ship with ease, so all this arrogance made him not suspect Tony taking away the Infinity stones from him.