John spider Sally (VERY DANGEROUS)
Do you have a picture or, anything? Your question is too vague for anyone to be able to answer it. But considering you saw this spider in Detroit and, appeared to be covered in Detroit sand I am assuming it s a motor city spider.
There are spiders that can easily blend to the color of your carpet or your walls, its truly evolutionary, but it takes time for them to do that.
to make this question somehow relate to travel- Detroit maybe it was covered in Detroit dirt after traveling around the city for a while
You really should have described the spider. It must be a Detroit spider?