When Does A Non Custodial Environment Become A Custodial Environment?

That is what the defendant pays 10,000 for a good lawyer to argue.

First of course it only applies to when they are questioned, and only when they are a suspect. It does not matter if they are in a custodial or non custodial environment as long as they are a suspect they have to have the rights read before questioning.

If they are not a suspect, at the time they become a suspect they have to have the rights read.

Of course a non suspect will not be in a custodial environment unless they are in it for a non related event, such as someone arrested for another crime, is in jail but they may be a witness of another crime, they are in custody, but do not have to be given any rights to ask them about the other event.

But as noted, a person who is the suspect can not be questioned even if not in custody. This is where the lawyers fight, at what point did the police questioning them as a suspect decide they were a suspect.

As for as a custodial environment, it can differ by state determiniation of the law, in general it is when and if a normal person under similar circumstance would believe they are in a custodial environment.
As long as they believe they are free to leave ( even if they are not) but if they believe they are, it is not considered a custodial environment as to the suspect.

** This is per GA court system as of 4 years ago. I arrested and questioned people for various thinigs then.