What to do if you see a kidnapping ....

[stopdiggin]

7. and not sure if they are 'required' to provide ID to anyone (and everyone?)

passing by on the sidewalk. Further, it would seem that these 'agents' have been give full authority (and approval?) to go incognito - without markings or identification - by superiors and higher authority. If that standard is to be overturned, it would need to happen in the courts or halls of congress. But it isn't going to happen on the street.

Record. Look for helpful detail - make, model, license, type of uniform and equipment. Ask questions - (if you're bold, and feel safe in doing so).
But, using caution (and a decent distance). Your right to 'intervene' or 'interaction' (as a witness/observer) is very limited - and could very well land you with charges and arrest yourself.