from the game guides
https://aw.my.games/en/about/game-guides/spotting-and-camouflage
“Spotted” Indicator – an icon that appears on the screen whenever your vehicle is spotted. This indicator is present for all vehicle classes but while for most of them it activates only 2.5 seconds after you become spotted by someone, it is instant for the majority of the Armored Fighting Vehicle class, allowing it a bit more time to retreat to compensate for its fragility (the bonus is in fact granted by the Reckon Package module that many AFVs have).
my bitch is the time is greater than the 2.5 as stated.