Officers need something else to identify them quickly
to other officers--like a badge worn around the neck which they can pull out, or even a T-shirt they can strip down to (superman?)
But it's in the interest of the public to have these officers armed and available, even when they are not on duty; our protection is in their hands.
My protection is in my own hands.
This subject is covered heavily by a decent police academy and any ajency with a decent post academy training curriculum.
We had a event last week where about 12 of us took down 3 in front of an apt. and we had one in plain clothes across the apt common area (all of this was outside.) His number one concern was not getting shot by a fellow officer and he was especially gratefull that no student officers (who would not konow him by sight) were on the scene.
It was all planned out and just before we could move in the subects started coming out of the apt..
Best thing to do when in plain clothes, off duty is not to get involved in the takedown or unless unavoidable. Period end of story. That being said, sometimes you have to act.