Attribute "Mark as used" can get set to “yes” in the properties pane of a nanoflow as well as for a microflow. This indicates that the flow is called by some part of Mendix that the Modeler cannot see, usually from a widget.
It leaves no trace as to where it is used and it can be pretty hard to impossible to find in some cases. It would be nice to have another string-attribute added to the modeler where the callers can be desribed.
In the where-used-field you can add freeformat text like “1) in widget xyz 2) in widget abc”.
The where-used-field should be filled if “mark as used” is yes, empty if no