Answer:
Prepositional phrase gives us the position of a noun.
1. Under the bed
Tells us the position of the Monster.
2. At the recital
This modifies where Roy was at
3. At the weeding
Modifies the position of where the friends could have been found.
The others are not prepositional phrase like
what lies below the Earth?
If it were written The Earth lies below the sun.
Lies below would have been a prepositional phrase.