This spot is quite vulnerable from Water Treatment surroundings but ennemies will not have great angles to stick grenades and you can move to avoid them. You can keep your weapons out in case someone makes the mistake of jumping there. It is not tricky to get there so if you are spotted, you have to expect people to come and fight. However, if other teams are busy fighting in Water Treatment, they will very likely not notice you and it is quite easy to third party them from that location. When you are at the same location of that screenshot, keep going down to the lower level where you will be invisible from the people moving along the cliff.

You can then lock yourself in the crouch position to be less visible.