It's a warning to alert you to either an attempted theft (ie; someone has tried lifting the car and set off the tilt sensors), or a possible fault in the immobiliser system (if your key doesn't start the vehicle, it may be a system fault or some nefarious scroat has cloned a key and unregistered yours)
Alfa Romeo Mito security lock warning light - this warning light (or symbol on the display) comes on (on some versions together with a message in the display) to indicate the failure of the Alfa Romeo CODE or alarm system (where provided): if this occurs, contact your nearest Alfa Romeo Authorised Service as soon as possible. Break-in attempt
If this warning light flashes or, on some versions, if the symbol appears in the display (together with the associated message) this indicates a break-in attempt. Contact an Alfa Romeo Authorised Service as soon as possible.
Had the same on the moto when it was locked / alarmed while passenger door still closing.. so pasenger door did not lock and then the passenger opened the door .. alarm went off.. closed door and re-armed alarm.. next time we got in the car that symbol was on with break in attempt detected (or words to that effect).. went after being driven for a minute.