Masked battery

a battery artificially concealed until required to open upon the enemy.
(Mil.) a battery so placed as not to be seen by an enemy until it opens fire.

See also: Battery, Masked

References in classic literature ?
The presence of these cannibals affected them no more than the soldiers of a masked battery care for the ants that crawl over its front.
They made, in fact, a masked battery behind the decorous office blinds, and there lay in wait as simply excited as children until at last the stem of the luckless Wetterhorn appeared, beating and rolling at quarter speed over the recently reconstructed pinnacles of Tiffany's.
My first observation was by way of opening a masked battery.