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.