Why is my Nokia Lumia 900 showing the volume every time I unlock it?

Whenever I press the unlock button on my phone, the bar for the volume comes down from the top of the screen, as though I've pressed one of the volume buttons (which I have not). It's a fairly minor thing, but it is annoying, as that bar covers the battery indicator. So... does anyone know what's up?

Is it also maxed? All the way up or down?

Is it possible that it the physical button is jammed?

Have you tried a Reset to rule that out?