The EXPS3 is described as:

A holographic sight. Swap reticles while aiming.

It’s found in north west Caimanes province, in a farm off the road from Ocoro province. There’s no resistance here. The accessory case is inside the farmhouse.

With no resistance, you just need to get to the farmhouse which you could fly to or drive off the road to the south of the farm across the field. There’s a sweet all-terrain buggy you can drive away from the farm afterwards though!

This sight has a simple 1.0x zoom and two reticles. It’s similar to other lightweight red dot sights but I do find it’s shape quite bulky, and blocks a lot of the view on screen when you ADS.

My favourite red dot is the Panoramic Sight (located in Itacua) for its simplicity and light design, but this is an easy scope to collect so you may want to give it a go yourself.