Black keyboard randomly appearing, sometimes displaying my login passcode

iPad 3, iOS 8.1.1 (12B435).

As the title says. A black keyboard randomly appears, sometimes displaying my login passcode as four digits at the top left of the superimposed keyboard. It happens any time, during any task (usually not entering text where the keyboard is required). I always use a quality bluetooth keyboard and only occasionally use the screen keyboard for selecting eMoji characters. This is achieved by hitting the relevant bluetooth keyboard button to call up the screen keyboard.

It's difficult to make the black random keyboard disappear. The usual keyboard-call button doesn't do it, nor mostly does pressing the home button. When I hit the home button, it works as it should, but the black keyboard stays on top and then half-covers the app icons. See two of the screenshots below, one which shows the (now-blanked-out)four-digit code enclosed in quotes.

I'm pretty concerned this is a security issue (and I'm far from being a paranoid person), especially since my login code (which I've changed) appeared for all to see.

Any ideas? Thanks. :-)

Message was edited by: goltho: Screenshots added.