[xoa]

desdiv's document is a great read. If you're referring to Miranda specifically though, I think a key part there might be "If you are not in custody[...]". The Miranda v. Arizona 1966 SCOTUS ruling had to do with violation of 5th/6th Amendment rights against compelled testimony and right to counsel. If you are arrested and interrogated, obviously you are under the government's direct power by definition, and if law enforcement fails to make clear to those arrested that testimony is not something they can actually coerce then anything they gain might get tossed on constitutional grounds. It's as much for the protection of prosecution as anything actually.

But nothing in the Constitution prevents you from volunteering whatever the heck you want to anyone, including someone from the government, even if it would a completely stupid decision to do so. If an agent/officer just asks you a question that doesn't mean you're in custody. They can legally draw inferences from any of your resulting answers/non-answers, just as they could from 3rd party witnesses, though how well any of those hold up in court is a different question of course. Affirmative assertion of rights is the neutral course there, it doesn't mean anything except that you recognize the situation is serious and want counsel to help you navigate it. It's not an admission of anything, it puts things on hold.