Actually,Satellites are used to Monitor missiles within their bunker or such.But nobody monitors where a trailer is going 24X7 and follows it.Countries generally disperse their assets in there is any need of launching such missiles.There are ways to monitor such activities and "HUMINT" is best way to monitor.
What generally a country perform is keep an eye on any known Missile Base and follow its activities.
And another thing,what do you think a country could possibly can by following the Missile Carrier??
Can they stop it??NO.
Can they mount an attack against it??NO.because if battle is going on at Indo-Pak border,Missile Carrier is possibly somewhere in Madhya Pradesh.
They can launch an missile attack against the base,but that can be detected long before it can reach the target and even before reaching,Missiles can be fired.
All these are quite known facts.