Why is a Walsh Bus Driver asking students to check under their seats for suspicious packages?

My neighbor tells me his son came home from school Friday and that when he was on the bus, the driver asked him to look under his seat for any suspicious packages.
Why would a bus driver do that? 
If there was an issue shouldn't the bus driver have checked the bus before the students got on the bus? Shouldn't the student have alert the parents?
Did any other parents hear the same thing from their child?
Patch can you check into this?