This, how you may remain hidden, is mostly dependent on objectives of given mission, but regardless of the situation, keep in mind a few important things:
- Try not to cause unnecessary confusion by avoiding contacts with enemies and not informing opponents about your actions as long as possible.
- Keep special precautions during exploring areas, where uninvited characters are not welcomed.
- Cover your tracks by dragging stunned or killed enemies to secluded places. You can find various cabinets and boxes where you can hide bodies, very useful.
- Aim at obtaining proper disguises, so you could more confidently enter the restricted areas.
- Use elements of the environment, hiding behind covers as often as possible and moving smoothly between them.
- Use the area map as often as possible (if it is available) as well as Instinct, thus tracing enemies' actions and surprising them or avoiding them.