Absolutely right, A is colloquial shorthand and not grammatically proper. Adding who improves the text but doesn't fix it; there's still an unsemantic tense use. How can you close the curtains if you've already left the office? Both B and C are acceptable and correct remedies.
B indicates you should, as part of the process of leaving, close the curtains.
C specifies that the curtains are to be closed immediately before exiting. The use of present tense tells us that the two actions happen at (effectively) the same time, and logic tells us that the curtains must be closed before you've left. Whether or not closing the curtains is part of the process of leaving is technically ambiguous but irrelevant.
(If you are the last person to leave applies in both cases, obviously.)
You're also spot on with the addition of the; it should indeed read the curtains. Technically speaking, without the it's unspecified which curtains should be closed, though the meaning is readily evident without the article.
Bonus alternative: you might also fix the sentence like this:
Please close the curtains if you are the last one left in the office.
Here, one need not be leaving to be obligated to close the curtains.
Best Answer
No, they do not quite have the same meaning.
"You are getting close to..." implies that the person is moving. For example, they are searching for the sign, but they are not "getting close to any".
"You are close to..." implies that the person is still. They may have been searching for it, but they are not currently "close to" any signs.
Also, your sentences don't make very much sense. Here's what they should be replaced with:
For example, If my friend was searching for a sign that I placed, and was walking around and could not find it, they might ask me for a hint. So, I would say "You are not getting close to the sign". (My friend is moving, and is either getting closer or farther from the sign as he moves)
For the second sentence, if my friend was about do do a flip, and wanted to make sure that the sign was not in their way, they could ask if they were close to it, to which I would respond "You are not close to the sign". (My friend just wants to know his surroundings, he's not currently trying to get closer or farther away from the sign).
"getting closer to" should only be used if the subject is moving and wanted to know where the object was in relation to them. "close to" should be used if the subject is not moving, and doesn't really care about exactly where the object is