Re: Grab your thumb, yes. Chalk your tires, no.
The tyre chalk ruling is different.
When I park my car, in a place where I'm allowed to park my car for an hour, I'm totally innocent. I haven't done anything wrong. For the next 60 minutes, I still havent done anything wrong. Since I haven't done anything wrong, they are not allowed to put chalk marks on my tyres to see if I _will_ do anything wrong.
Just like the police wouldn't get a search warrant to check your phone to find out if you have committed a crime. They must know of a crime, and have a reasonable belief that the phone contains evidence. In the parking case, there is no crime (yet).