My brother described above was
Mr. Furtive Movement. When we were little we called him "Flinchy" when we wanted to be mean. He looked "suspicious" just standing still in front of our mother's house waiting for a ride. If he noticed anyone looking at him, he would get even more nervous and shifty-eyed, start to whisper to himself, fiddle with his pockets, keys, cap, whatever. Imagine how often he was brought to the attention of security guards and police in malls, stores, parks. Which made him even more nervous about going out and about. It was a vicious cycle.
I think I wrote here once about how he once ran out of a store with some batteries in his hand, after waiting patiently in line, his money out, to pay. Because he saw his bus coming across the street and knew that he would get yelled at if he got home late again. By trying to do what he thought was the right thing, he ended up in the hands of the police.... again.
Another time I was chatting with a friend who worked in a photography store, and a scruffy-looking suspicious character walked in. He walked slowly and aimlessly through the store, hands in his coat pockets, looking at things a little too closely, nervously glancing around like Shaggy in Scooby Doo. My friend looked alarmed, whispered, "Oh my god," and reached under the counter to push the security signal. I quickly put my hand on his arm and said, "It's okay. He's not going to take anything. I know him."
It was my brother. My friend was gobsmacked because 1) he was sure he was about to get robbed at gunpoint, and 2) he was shocked to learn that I had a "retarded" brother. It was not the sort of thing you told your friends if you were trying to be a hip and cool 20-something.
That time, he did not get detained, arrested or roughed up. It was just a fluke that I was there.

When threads like this come up, many folks, esp white folks go into denial.
Some deny the reality of racial profiling; "Oh, c'mon, black men don't get pulled over for no reason more often than any other group. If you don't do anything wrong, you don't have to worry."
Or they decide that we are exaggerating or lying about our experiences; "You folks are always crying racism-- you are really the bigots, the way you always suspect white people of racism. Black police pull over white people and we aren't complaining. The police are just doing their jobs."
Or what is worse, they try to
justify mistreatment by police. "Well, young brown-skinned men commit more crimes, so of course the police will target them. What do you expect the police to do? Pull over law-abiding white people?"
If you were the mother of a dev. disabled black kid like my brother, what would you do to keep him out of police custody? Lock him in the house 24-7? Pay someone white to escort him everywhere he goes?