Not sure if it's been discovered or not, but I found a bob while playing Nightfall. Spawns on a path at the beginning and runs into the main view for a second.
http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=513072 6&player= Davoony
Delete the space between = and Davoony.
The Bob is hard to see, but I tried to get a view to show the area he was in. It's the first part of the mission, right after you assassinate the Elite and continue on to the big opening with a lot of Covenant. He spawns on the far left, on the white path. View high res and look at that area, you'll see the dark colored figure.
[Edited on 10.06.2010 11:19 PM PDT]