I thought a field was only used in one place - on a particular layout - and I wanted to change how it worked so I performed a Database Design Report to confirm this.
When I looked at the entry for that field in the DDR, it indeed indicated that it was only used on one layout.
I did not realize, however, that it was also referenced in another calculated field.
I created a test db and DDR as an example. In this case, FieldB1 and FieldB2 reference FieldA, and FieldC returns FieldB1 or FieldB2 depending on the value of FieldA. In my working database, the fields I wanted to change are the equivalents to FieldB1/B2, but I see that I didn't need to build this complex an example to demonstrate the issue.
I can imagine that the designers of the DDR would be reluctant to add yet another column to the Field table. Perhaps one of the columns could be hijacked, e.g. rename "In Scripts" to "In Scripts/Calculations" and distinguish between In Scripts and In Field Calculations with bold headers as is done in the Options column.