It is about stress in the sentence. "er" does not stress the specific relative location, it simply means here/there as per context - as explained by vam1980. If you want to stress the specific location you can replace "er" with "hier" (here) or "daar" (there) which gives it a crisper meaning.
The difference between ‘already’ and ‘yet’ is pretty subtle. See http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00334.html for some discussion. I'm not sure how closely this matches the difference between ‘al’ and ‘nog’ in Dutch.