Question:
During movement phase a friendly plane flying CAP (since the initial phase) could patrol intercept an enemy plane that has just (during the movement phase) flown a CAP mission into the same hex as the friendly CAP mission. Is this correct?

Answer:
Yes. The plane(s) on CAP may (but are not required to) react to any enemy mission force that enters their hex, including an enemy CAP mission. Note that in your example any of the friendly air units that do patrol or intercept have then finished their mission and return to base. The enemy air units that survive this encounter remain in the hex to perform their CAP mission (that is, the friendly air units have completed their mission but the enemy air units have not).

Source:
TEM 59/60