You would be wrong. Walt Disney World does not have ANY buses that travel from theme parks to Downtown Disney, at any time for any reason.
Guests wishing to travel from any theme park to Downtown Disney must got to any WDW resort and transfer to a DTD bus.
What you MAY have seen, although I didn't think the marquee indicated Downtown Disney, is the Hotel Plaza Boulevard bus - the privately-contracted buses that transport Guests between the seven non-Disney-owned Downtown Disney area resorts and the various theme parks.