Has anyone else had ArcGIS 10.4 add a drive letter when opening an mxd. The correct path is something like \P\GIS_Data\ND.gdb, and it got changed to \P\j\GIS_Data\ND.gdb. It doesn't happen often enough to really be a problem, but when it does happen its frustrating.
If "P" is a server/machine name,try seeing if it likes \\P\....
If it is a relative location from the current location, may it would like .\P\.... (notice dot slash at beginning)
Otherwise, a snapshot of the resources for the mxd woudl be helpful. May in ArcCatalog (with he ,mxd closed) if you right-click on the mxd it has an option to "change datasources". If you use that (can cancel without changing anything) you will see how the .mxd is referencing the data. My give you some clues on the issue.