What not free to leave (and thus, warnings required) looks like is a fraught and fact-specific question.
Consider an already-sentenced prisoner called into a meeting with cops about some different crime. Most courts, incorrectly IMHO, say the prisoner lives there and it's like being at home, so it is not a "police-dominated atmosphere" & without more (cuffs, etc.) he is obviously free to go so no warnings required. Get real.
None of this is obvious. That's why you need a lawyer. /2