"rest"room etiquette

alright, so I am starting this blog while thinking about a question I have been asking myself for quite a while now.  So this has to do with restroom etiquette... It is not about if you place your toilet paper over or under...its not about how many times you flush when you go number two...its not about people using the spray in order to get rid of odors...its not even about whether people put the bathroom seat back down when they pee so that you dont fall in the damn thing when its dark, although this really pisses me off and makes me raise my fist in anger.
This has to do with those times you are in the restroom at public spaces, at other people's homes and especially at work.  I have found myself in the restroom a couple of times...well more than a couple of times...well actually let me paint a picture.  I go into the restroom.  I place a seat cover on the seat.  I pull my pants down and finally take my seat.  What a relief!  Doesn't matter if you are doing number one or two.  you are at the point when you are pushing, wiping, or getting ready to zip up and then you have someone knock on the door.  My question really is: Why do people knock on the door when you are in the restroom?  What are you supposed to say? Am I supposed to say Its busy? or maybe I should say hey, Im almost done.  or maybe I should say its gonna be a while.. Should I say come back in a little while.  Maybe I am the only one that finds it weird for someone to knock on the door.  What are they going to do when they knock and you respond?  It's not like I am going to come by and open the door welcoming them in.  So really I'm just saying maybe the next time you are trying to figure out if someone is in the restroom look and see if the lights are on.  Secondly, you should probably try to open the door without being too loud so that you can see if its locked.  You dont want to scare people that way.  I'm just saying have some restroom etiquette and don't make the restroom user feel uncomfortable by knocking on the door. its too much pressure to make a decision about whether or not to answer back or what to answer while you are trying to rest in the restroom. 
My strategy is to just go for the door knob.  I'm just saying you should be courageous.  Grab the door knob, turn it and push the door.  I recently did this twice in one day because I expect people to lock the door.  Unfortuntely, those two times the same person was not able to lock the door and I scared them a little bit.  Thankfully, this doesnt happen too often.  if it does happen at least you have a good laugh about it later on.  Oh, but I do support people knocking on the door at public restrooms when people take too long and u gotta pee. 

So, am i the only one that has thought about this?