Sherlock threw down Moriarty's body before Watson arrived. When Watson
approached the hospital, Sherlock was busy keeping him behind the small
building so Watson can't see the dead body. Then Sherlock kept him busy
until the biker arrived. He then jumped into the truck which was waiting
at the bus stop - out of sight of Watson - and was obviously filled
with garbage bags. Watson runs towards the scene, gets knocked down by
the biker, who was part of Sherlocks homeless network, and now - half conscious, seeing a dead body on the ground -
believes what he just had seen, that Sherlock jumped of the building
into his own death. Sherlock leaves in the back of the truck in the
meantime.