When do you have to read Miranda before questioning?

Alot of officers and attorneys believe that you have to read a suspect his rights before questioning him if the suspect is not free to leave. This is absurd. Check out this question from one officer about this....

Question
"An incident arose the other day that made me think of your class.

An Officer stopped a driver and passenger for a valid violation. (Should be noted there was no need for him to wait for the violation to occur due to the reasonable suspicion of the two looking into parked cars windows in an apt complex parking lot at 0300 hours.) No crime was committed at this time. The Officer called for a K-9 to check the truck and the dog alerted to the interior of the truck. A subsequent search revealed a meth pipe between the passenger seat and the center console. Neither would admit to owning the pipe. Officer arrested both of them for possess drug paraphernalia.

The problem arose when he was told he violated the constitutional rights of the two by not reading them Miranda before asking them about the pipe. Then another arose when he was told the arrest was not valid because he could not prove ownership of the pipe, neither would admit.

Answer
First -- and as an aside -- The USSC in MARYLAND V PRINGLE held basically that if evidence is discoverd in the passenger compartment both the driver and passenger may be arrested. (That doesn't mean they will be prosecuted -- you are going to need an admission or fingerprints to get a prosecutor to prosecute probably.)

Second -- I need to know whether the two were "restrained in a significant way" at the time they were questioned -- were they handcuffed or in the back seat of the police car? If not, then I would think there was no custody and any statements they made would have been admissible.

Best -

DA