There is another way to get to Marchenko's Killswitch.
The first opportunity is already given in G.A.R.M. Facility. In hangar 2, in the tent to the left of the lift, there are several spins in the back corner. Jump on it and look for the button between the spint and the tent wall. This button opens, in the spint, a secret compartment with Marchenko's Killswitch.
Attention!!! For this, there is no longer a suitcase in Mission 15, with the Killswitch.
G.A.R.M. Secret Placement
Hangar #2 location.