Be careful with trigger once because it has a tendency to f things up as soon as there's multiple instances.
Your current solution isn't far off though, all you have to do is to invert your logic.
Enemy has LOS -> Start timer lost
(no need to stop the timer on LOS lost or anything)
As long as the enemy has LOS, the timer will run indefinitely. When the enemy has lost the LOS, the timer will run out after 1 second and the enemy will move to the return point.