This is another illustration of why I recommend avoiding global variables except as a very last resort. There are very few situations where it is necessary to use a global variables. Sometimes it can seem easier to use one, but often that will come back to bite you later, and as illustrated here the problem can be very difficult to track down.
I wonder if you might have a form with the properties set to create global variables. The default is fileglobal, as shown here, but if at some point you set this to global that could easily create global variables you weren’t aware of. (This setting was also available in Panorama 6, and would have been brought over.)
You could use View>Open View to search for text editor objects that used the TixCreditCk variable. Use the Full Source option.
You can also get rid of a global variable with one line of code.
Of course that still leaves you with the mystery of how it was created in the first place.