I live in a relatively criminal city, on a street that's not a 'through street' so there's no reason to come down it unless you're going to a house on it.
We have young men who ride bicycles around the neighborhood at all hours of the night. I'm pretty convinced they're often up to criminal activity, mostly theft.
They could be looking for things to take, or to call others and notify them of targets.
One time, I saw a cery 'suspicious looking' bike rider and asked him to say why he was there. He said something about visiting his friend up the street. I got in my car and did similar to the story, following him slowly. It was clear quickly he wasn't actually visiting anyone. He rode his bike past the house he'd identified and then back to the main street. At first there was a bit of trash talk then he didn't want to talk, and walking his bicycle, he was on his cell phone for several minutes clearly talking about this.
I continued to follow him for several minutes and blocks. Then he stopped at an open area, and waited. I didn't have a cell phone to call the police to check him out.
A man who looked in his 30's rode a bicycle up to him - clearly who he'd been talking to on the phone, looking not like a friend or relative but a criminal contact, and I forget his comments but it was basically some sort of defiant trash talk or threat. Finally the two of them rode off - I'd been hoping a police officer would drive by but none did - and I left at that.
The next day, there was a knock and a third person, a young hispanic guy, was there to try to be threatening acting like a drug crazed gang member, saying not to bother 'his friend' and all kinds of profanity and gang allusions and threats, I remember saying he 'is diablo', Spanish for the devil and such. Finally he wasn't getting anywhere and walked away. I immediately called the police and then tried to find the kid, but he was gone.
It wouldn't be that hard for street gang types like this to drive by and shoot into a window. No way they'd get caught.
A few days ago, similarly, I went out for a minute at 2:30 AM, ando found a suspicious bike rider slowly going on the street. Got in the car and followed him, again no cell phone. Followed him probably 10 minutes as he went a long way to some whole other neighborhood - again it was clear he had no business where I found him, riding away from the main street and then back to it. Finally a police car drove by and didn't see me wave and I decided to drive after him to try to flag him down.
When the bike rider saw this he did a sort of sarcastic wave, and then I couldn't find him.
I can see why there's a need to confront these people. There's a difference between this and the other situation in part because of there being not a through street.
(It actually is a U street, so if someone is going up one side the only reason is to go to a house; riding all the way around back to the main street shows they weren't visiting).
Recently a young career crimnal drug addict woman was driving a car I suspected was stolen. I called the police with the plate, they confirmed it was, and quickly came and arrested her with the car, and took a report where I confirmed I saw her driving it. She had gotten out of jail weeks before after a parole violation for a stolen car; and already was wanted for another stolen car; and this was another stolen car.
Her only punishment - the downside of California's budget problems letting non-violent prisoners out - was a probation violation putting her in for 4 months. No charges for the new stolen car. The district attorney's office said that she could make up a story hard to prove was false beyond a reasonable doubt so they just violate their probation if that's an option.
I discussed the crime problems with the assistant district attorney and her comment was 'you should move'.
The police had also suggested I might want to start a neighborhood watch...