I'm assuming this will be one of those things that is "undefined", but I can't seem to find a concrete answer from google.
Let's say in my vertex shader I have:
layout(location = 0) in vec3 vPosition;
layout(location = 1) in vec3 vNormal;
layout(location = 2) in vec4 vColour;
But there is nothing buffered to location 2 with glEnableVertexAttribArray() or glVertexAttribPointer(). Can I expect the value to be anything particular?
I was assuming for a vec4 that it would be along the lines of {0,0,0,0}, {0,0,0,1} or {1,1,1,1}, but in my case it is {0,0,1,1}.
When I previously used glBindAttribLocation() to specify the locations, it defaulted to {1,1,1,1} on 4 different machines using 3 different operating systems (ubuntu 12.04, windows 7, and ubuntu 10.04).
Is it safe to assume the value will be {0,0,1,1} across machines? or was this simply a coincidence?
Can I expect the value to be anything particular?
Yes, it is a particular value.
Assuming that you correctly turned off the attribute array (ie: with glDisableVertexAttribArray on that attribute index), the value you get comes from the in-context vertex attribute, as set by the glVertexAttrib suite of functions. These are global context state, not stored within the VAO.
By default, they all start at (0, 0, 0, 1). However, if you have rendered with a particular attribute using an array in that attribute index, the context value is effectively destroyed. So you should reset the context value if you want to use it.