From the Scripting Manual
procedure OnFlagGrab(ID, TeamFlag: byte; GrabbedInBase: boolean); Parameter Info: ID (Byte): Player ID of the player who grabbed the flag. TeamFlag (Byte): Team ID of the flag which was grabbed. GrabbedInBase (Boolean): True or False if the flag was grabbed from its original spawn point. Description: This procedure will be called every time a player grabs the enemy flag.