Well, given the fact that backpack was found in pretty busy neighbourhood, with high school across the street after more than three weeks from Mariam's diasappearance tells you something: people just do not pay attention anymore. Ol' little ladies with keen eyes are not around any more. It is just hurried and busy people not caring at all about what is going on around them. I strongly believe this is the reason why whoever had done this to Mariam, unless someone "talks", wiill never be caught.
This is interesting question. I personally had only few direct encounters while sleuthing. One of them was when I went to library, but to my disdain they were totally not interested in answering my questions. Second one was when I was observing parking lot on south side of the school one morning around the same time when Mariam disappeared, to determine how many students really take the route that Mariam apparently took: one of the teachers ( I assume he was the teacher - he went into school ), looked at me in a kind of a strange way. There was not many of them at that time arriving to school, but all others did not pay attention to me at all, except for this one. Also, there was a little bit of awkward communication here on the thread when my motifs were questioned by one of the participants who also claimed to be neighbour. I cannot remember nick name, but if one goes far back at the beginning one could find this odd comment.
I have to add here that I had a chance to speak directly with former VP at FHCI, whom I met by chance, and who also agreed with my suspicions that something more sinister happened here. He enroled Mariam to FHCI, met her personally, as well as her parents and confirmed that Mariam was very ambitious, well composed young woman, with high aspirations and obviously many talents, not only for math and science, but languages too.