A SOVIET submarine operating in the Barents Sea fired a cruise missile through Norwegian airspace into Finland's northern bleakness recently. Officials in Washington as well as in Oslo report that it probably was a stray, apparently unarmed, fired by accident during an exercise. One hopes so -- although the thought of an accidental firing is not exactly comforting.

It would have been reassuring, in any event, if the Soviets had made the first report of the shot and if they had apologized promptly for it. As their downing of the Korean airliner indicates, the Kremlin gets awfully urgent and tight when Soviet airspace is innocently violated. Its concern for the tenderness of borders has a certain one-way quality to it.

But let us examine another thesis: Suppose it was not simply a test shot that flew off course. Suppose it was also a shot fired, or used, for a reason: to assert a Soviet claim to the operative sovereignty of the northern region lying near the Soviets' sensitive Kola Peninsula bases, or to lean on Norway, which has been the object of a nasty Soviet political campaign designed to thin its working ties with its NATO partners.

The Soviets can have no worries about neighboring Finland, an independent and neutral country made permanently deferential -- by defeat in war and then by treaty -- to their strategic interests.

In recent years, they have repeatedly penetrated the airspace, waters and the actual soil of neutral and unoffending Sweden. Military preparation and political bullying are the apparent purposes.

In Norway, the Kremlin has been doing its best to cultivate nuclear jitters by claiming that the Reagan administration seeks to make Norway a "springboard" for nuclear attack on the Soviet Union. It is urging Norway, which has long had a policy of barring foreign troops and nuclear weapons in peacetime, to restrict further its participation in NATO affairs. As recently as Nov. 27, Moscow alleged that Washington was considering deploying in Norway "cruise missiles with conventional warheads, which can be reequipped with nuclear charges" -- precisely the kind of weapon a Soviet ship apparently let fly over Norway a few days later.

Throughout Scandinavia there is a disinclination to make a dark reading of this latest incident. The Finns noted only the passage of a "flying object" and doubt that it can be found. The Norwegians waited to report it until everybody was back from holiday. It is, at most, a small affair, but it briefly lights up a northern landscape shrouded in more than one kind of winter darkness.