johnwasser:
After playing with this string it appears that Text Edit prefers to break lines at after the ':' of a Letter Colon Digit sequence. It doesn't break at Digit Colon Letter, Letter Colon Letter, or Digit Colon Digit. This means that there is no "hidden" character in your string, just a pattern that Text Edit prefers for inserting line breaks when soft wrapping text.
That's it; You're a genius!
I played around with this question last night until around midnight...googling ever possible explanation...I even typed in several strings by hand to see if it had something to do with pasting it over from the serial window. No difference.
I guess I already knew there were no hidden characters after looking at the hex of the file contents. But it was still kinda driving me crazy.
I then downloaded about 5 different text editors to see if they would do the same thing. The first four editors did not have virtual line breaks inserted as TextEdit does. Finally, the fifth editor I tried, un-creatively named Tex-Edit, also had virtual line breaks added...but it added them BEFORE the "DISI:" instead of after! Actually, nicer looking, but pretty clear indirect evidence that there are/were no hidden characters.
Thanks for working on this puzzle with me. Next we should tackle peace in the Middle East.