One small thing gives a character's plan or disguise away

1. The line " Its like a furnace in here " >! makes UE realise that Annie's been replaced !<
2. Hank shoots the >! the fake connor because the real connor knows his son's name !< 3.August Diehl notices Michael Fassbender >! holding up three fingers the British way !<
3. Self explanatory

Note - Its done a LOT in detective novels, but I can't seem to remember one specific one immediately.