I'll tackle this one. The red ribbon is there to remind someone that whatever it's attached to needs to be removed.

Whenever a plane is parked, it's possible that something has been attached to it that shouldn't be there during flight--a protective cover, external power supply, etc. The red ribbon stands reminds the pilots/ground crew to remove the item.

As an example, one common item used is a pitot tube cover. Without getting too technical, the pitot tube measures airspeed. If this is covered during flight, it can have terrible consequences. The cover has the red "remove before flight" tag on it so it's not forgotten.