realistic possibilities:
tourist put it down by accident, forgot.
tourist put it down intentionally to make a statement
domestic intelligence placed it there to make a statement or cause a stir
foreign intelligence, same reason
it belongs to secret service member or wh staff
it belongs to bidens or some other politician
there are plenty of ways it could happen, and sure they search everyone but its not a pat down/ strip search.
we just don't have enough info.
but the people "in charge" do… that's the hard part to swallow.