There are 3 RUSH upgrades in the game.
#1: Cell Blocks. After acquiring the Batclaw, pull down a vent cover below and to the left of Cell Block C. Enter it and follow the short path to acquire this upgrade.
#2: Industrial. Directly below the dead engineer. You'll need the gel launcher and the line launcher to access it. To the left of the dead engineer, scan and blow open the floor, then drop down and line launch across the spiked fence.
#3: Industrial. After the fight with several thugs on electrified floor panels (this is in the foreground of the room with the pressure-plated door), go left and scan and blow-up the grate. Drop down and follow the short path to the right, to then scan and pull down the barrier, after which you can access this final upgrade.