In AUTO mode Plane will follow a mission (a set of GPS waypoints and other commands) set by your ground station. When re-entering AUTO mode Plane will continue from whatever mission item it was last doing, unless you have reset the mission. If the mission ends with an item that does not continue indefinitely (like LOITER UNLIMITED), an RTL will be executed.
When in AUTO Plane will by default allow the pilot to influence the flight of the plane by using “stick mixing”, which allows for aileron, elevator and rudder input to steer the plane in a way that can override the autopilot control. Whether this is enabled is determined by the STICK_MIXING option. By default stick mixing behaves the same as FLY BY WIRE_A (FBWA) mode.
“Home” position is always supposed to be your Plane’s actual GPS takeoff location:
It is very important to acquire GPS lock before arming in order for RTL, Loiter, Auto or any GPS dependent mode to work properly.
For Plane the home position is initially established at the time the plane acquires its GPS lock. It is then continuously updated as long as the autopilot is disarmed.
This means if you execute an RTL in Plane, it will return to the location where it was when it was armed - assuming it had acquired GPS lock.
Consider the use of Rally Points to avoid returning directly to your arming point on RTL