Preguntas sobre ejemplos de oraciones con, y la definición y uso de "Houseguest"

A: departure is the correct word.
You could also say "tail lights". It means they are driving their car out of your driveway to leave. haha
You could also say "backsides", because you are watching them walk away from you and are looking at their backs.

departure is not an idiom.
backsides, backs is not an idiom.
I think tail lights might be an idiom?