It should be relative to the .dae..
I am saying this because, I tried opening it using FBX Review which renders 3d.
FBX Review loaded the 3d model without the textures.
Moreover, the application does not know anything about textures. It just renders whatever is in the .dae
and all the information about the textures is inside the .dae...
My concern is that if some .dae file is created on Windows which refers textures as in my example, how will I be able to open the same on Mac (with textures), and vice-versa.
I tried changing the path to full path too in the .dae.