Definition of CUSTODY
According to the Austin Court of Appeals, in State v. Chupik, No. 03-09-00356-CR (Tex.App.--Austin 2011) that's exactly what it is.
It would seem that Randall Chupik was driving down W. 6th Street in Austin at one o'clock in the morning. A police officer stopped Mr. Chupik for weaving within his own lane (not a traffic violation). The officer testified at a suppression hearing that he had probable cause to arrest Mr. Chupik after waving a pen in front of his face. He also testified that Mr. Chupik wasn't free to leave.
After conducting the HGN test, the officer had Mr. Chupik perform other coordination exercises and asked him a series of questions. At no time did the officer tell Mr. Chupik he couldn't leave.
The Travis County trial court granted Mr. Chupik's motion to suppress on the grounds that the officer conducted a custodial interrogation when he asked Mr. Chupik questions after the HGN test and without warning him of his Miranda rights.
The Austin Court of Appeals upheld the trial court's decision on appeal so the state appealed to the Court of Criminal Appeals who reversed and remanded the case back to the Austin Court of Appeals.
This time around the Austin Court of Appeals held that a person is only under arrest (for Miranda purposes) if a reasonable person would assume, under the same circumstances, that he wasn't free to leave. The court also held that it didn't matter if the police officer felt the suspect was in custody. It only mattered if the officer told the suspect that he wasn't free to leave.
Nevermind the fact that we all know that once an officer begins conducting roadside coordination exercises we aren't free to go. Let's be real. Once that officer stops you, unless he tells you you can leave, you can't leave. It doesn't matter whether he says it or not.
This legal fiction that you're not under arrest if you don't know you're under arrest is yet another example of the courts bending over backwards to find ways to weaken the protections of the Fourth and Fifth Amendments.