The back cover has been pushed away and the display was bent too, as if someone had inflated a balloon inside it. I immediately suspected a faulty battery, but the tablet was working and holding the charge.
Actually a first sign of failure appeared one month ago on the display in the form of a liquid blob stuck under the glass:
But I though it was because of mishandling during holidays or just a failure after two years from purchase (warranty is over). The tablet never fell off. Another sign I have not recorded in pictures was a difference in backlight intensity in the border and one brighter spot above the apparent liquid leak.
These two alterations could be an early sign of upcoming battery failure in a tablet.
Next step: opening it up and getting rid of the battery, as soon as possible!
(For searches in Italian language: Batteria gonfia nel tablet Mediacom Smart Pad 930i. Continua nel post successivo.)