Does anybody know what the hell a "noncustodial arrest" is?

An oxymoron.

Some people consider citations noncustodial arrests because they bring the person under the authority of the court based on the same probable cause threshold of a custodial arrest.

Check out Carson v. State, 65 SW3d 774 (Ft. Worth 2001) -- a person's "actual submission to an assertion of authority" by appearing in court to contest charges is a noncustodial arrest. So if they get a citation or are filed on at-large but show up in court, that's considered a noncustodial arrest. (At least in the expunction context.)

I think a non-custodial arrest would include when a peace officer requests a blood sample from a DWI suspect receiving treatment at a hospital and immobilized due to injury. No rational officer would claim to have taken the patient/suspect into custody, but the first line of the DIC-24 states "You are under arrest for an offense . . ."

Is the immobilized suspect free to leave?

My bad. Suspect/Patient is in custody in that situation. Madeley v. State, 2008 WL 482337, Tx.App- Austin 2008

So, it's kind of like when you were grounded but could go to school or to a doctor's appointment? Lawyers sure can be silly.