IF (a) In Audio Engine Options, "Release device when Sibelius 7.x is in the background" is checked; AND IF (b) Sibelius is in the background (does not have focus) at the moment when the score file is opened THEN the Master Fader will not appear in the Mixer, and cannot be made to appear by clicking the "Show/hide master volume" button.
For instance, if one opens a score file by double-clicking it in Windows Explorer, then Sibelius necessarily does not have focus (is in the background) at that point, and the score will open without a Master Fader, as illustrated in the attachment (provided there was no interaction with Sibelius during the score-opening process that would bring Sibelius into focus).
In the attached illustration, the "Show/hide master volume button" (circled in red) is in fact in the "Show" condition, but no Master Fader appears.
There is never a problem when opening files from within Sibelius, because necessarily Sibelius has focus at that point.
I assume that this bug was introduced in Sibelius 7.1.2, and continues to be apparent in 7.5.