Actually some rear hubs (notably Powertaps since the introduction of the 15mm axle) have a drive side axle cap and freehub which can be removed without tools which is nice if you want to switch between Shimano/Campagnolo configurations for different bikes.

You can pop the freehub + cap off and pop it back on.

While you could theoretically introduce a mis-alignment by failing to fully seat the parts, closing the QR skewer should finish the job and put things back to normal.

On a more conventional hub I might suspect a drive-side nut that's not tightened all the way. That would allow cassette movement and make things nosier. Hub misalignment in the drop-outs is also possible.