I think the player swf is getting some manifest, smil, or json data that it is not entirely using. This is different than just what is in the page source. It is in memory, but no hint of the network resource from traffic or the decompiled swf.
Code:
http://cdn.videos.dolimg.com/channel_longform/gravityfalls/929066/be22aa-pgravityfalls_e1-06-f006_tdm_h929066_fprhq_v1080p23_a16x9-178_leng-h264h_aac_1920x1080_7840x160.mp4
It is probably not worth a lot of work locating the network source because they are obviously transitioning to flash access.