In Avengers: Endgame, future Nebula goes back in time to bring into effect the Avenger's plan. The minds of past and future Nebula are connected by quantum entanglement (maybe). So past Thanos gained access into future Nebula's mind via past Nebula's mind to see the future plan of the Avengers.
Now the past nebula has been living with past Thanos up till that time, all her life. After the future Nebula enters the timeline of the past Thanos, he shows up to the past Nebula saying that Ronin located an infinity stone. While saying so, he simply looks at past Nebula and feels the need to read her mind. My question is how this past Thanos sensed something is wrong and hence felt the need to read past Nebula's mind. This seems an enigma.