Hello guys thank you for all of your help.
I was able to find out where this DO was being set, unfortunately it was not in the program, it was in being set by a system variable.
As I suspected I miss typed the DO I was supposed to use.
I'm using a CR-35iA collaborative robot and it has a some DOs for indicating the collaborative operation status they are under the variable $DCSS_CLLB[1]
The bad part is that I may never be able to find it with the format DO[XXX] because it's only a number indicating the DO number.
Thank you all.