As your Plex Media Server transcodes content for Mobile Sync, it will be stored on your computer until a sync occurs. Once the content is synced to the device, the transcoded version is removed from your computer.
The specific location where transcoded content is stored varies based on your operating system:
On Windows, the content is located inside of the user directory.
%LOCALAPPDATA%\Plex Media Server\Cache\Transcode\Sync\
Using the Finder's "Go" menu, select Go To Folder... then enter the following in the dialog box that pops up:
In general, the location for the various Linux versions of Plex Media Server will be found under:
$PLEX_HOME/Library/Application Support/Plex Media Server/Cache/Transcode/Sync/
/share/MD0_DATA/.qpkg/PlexMediaServer/Library/Plex Media Server/Cache/Transcode/Sync
/Volume1/Plex/Library/Application Support/Plex Media Server/Cache/Transcode/Sync