This scope is described in the game as:]
Adjust the electronic zoom while aiming.
You can find it in southern Monte Puncu province, at a makeshift airfield atop a hill. There’s a “Gasoline Air Transport” side mission here to steal the plane. The accessory case is next to the plane, sitting on a crate.
There are 8 sicarios at the airfield, two of which are at the end of the runway. You can start by taking them out with a sync shot first to reduce the number to six, then take the rest out as you wish. Be careful not to blow up the plane if you plan to steal it, but it’ll return for another attempt later if you do.
This scope fits SMGs, ARs and snipers, and offers a low 2.5x – 3.5x switchable zoom. Personally, those two zoom levels are far too similar to be useful to me on any gun and I find there are much better alternatives, such as a Panoramic Sight for accuracy at close quarters, or something like the TA31H ACOG with a fixed 4.0x zoom for medium range engagements.
But maybe pick this up if you’re a completer finisher and want to get everything, or to try it out for yourself and see if you like it.