When files are specified without a full path, they will be created or opened in the "current directory" (aka "working directory") of the program.
If you're running ffmpeg from within Cmd or from PowerShell, the shell's prompt directly shows you its working directory (and therefore the working directory of everything you run from that shell).
For example, if you have:
C:\Users\CadenPolen> ffmpeg -i old.mp4 new.mp4
then both files will be in C:\Users\CadenPolen
, as that's the "current directory" of Cmd and therefore of FFmpeg, and a dir
would show both items in the file list.
(To change the current directory, you would normally use cd
in PowerShell or cd /d
in Cmd before running the program.)
If you ran ffmpeg from some other tool, that tool (like every other running process) has its own "current directory" set to something. (If it has an "Open file" button somewhere, the file selection dialog will open in the same directory. Otherwise, tools like Sysinternals ProcExp would be able to tell you the working directory of any process.)
For Windows specifically, in some cases, programs might actually do a %PATH% search in addition to looking in the current directory, so you might want to use where.exe outputFile.mp4
(it's "output", not "out-put") to possibly find it in one of the %PATH% locations. But as far as I know, this isn't done by ffmpeg, and isn't done when creating new files either (it only ever happens when opening an existing file).