Honour guard...

[Popsical]

...are they a retinue that must be attatched to the chapter master or can they act as a separate unit?

[Nabterayl]

Separate unit. Starting in 5th edition there was a move away from retinue units towards units that were only unlocked by certain HQ units, but otherwise completely independent of the unlocking HQ. You will never see an honor guard on the battlefield unless the chapter master is also on the battlefield, but they needn't move or act together. I assume you're asking because you can't find anything in the codex that requires the honor guard to be permanently attached to the chapter master (or vice versa) - you can't find it because it isn't there.