When a model is open for editing in ArcGIS Pro, there is currently no way to identify the source location of the model (e.g. the toolbox where it is saved). This information should be added to the model properties window.
Here is some further explanation of my situation that led to this idea... I opened a project that I hadn't used in several months. I was using models from similar toolboxes saved in several locations across my organization. I had one particular model open for editing in the project, but could not remember or identify what toolbox it was from. This model was not in the default project toolbox and was not listed as a mapped toolbox in the the catalog window.