A few more points to add:
I think that this would have to apply only to organic surfaces like dirt, grass, etc. It would be odd if concrete could give an indication of somebody's presence.
Certain weather conditions could make tracking more or less difficult. Rain might obscure tracks, making them more difficult to follow, whilst snow could make them far more visible.
There could be mechanics added for hiding one's tracks, or at least minimizing them. Not sure how this would work.
Perhaps people walking in single file could obscure their numbers? Think I'm reaching now.