Right, found it in the manual. MPEG-2 files are stored in the \MP_ROOT directory, in subdirectories named like 101PNV01, 102PNV01, 103PNV01, etc. Each subdirectory can hold at most 9999 files, when it reaches maximum capacity a new directory is created. I suppose, a new directory is also created for a different date.On the NXCam Mpeg2 SD is recorded on the same card but in a different folder.
Also interesting, that Sony uses \private\avchd\... for SD cards, but \avchd\... for MS cards and for the FMU. Maybe \private is a Panasonic thing? But Canon uses \private too.