Thibaud de la Touanne
Sorry, can't provide the official answer, but I don't see how it should not raise the alarm, as the rifle shows the alarm icon.
But, the sniper is still not spotted automatically when firing.
And I still think that it's a good ability as you might especially want to use it, when the alarm is already raised in which case you can even stop two enemies from entering that round - this may really save the day.
Thanks Christian, your answer is correct.