A bounced file is NOT referenced in Logic, unless you bounce to PCM (AIFF, WAV, SDII) and you tell Logic to add it to the Audio Bin. In which case it will be referenced in... the Audio Bin!
With MP3, this is not so, mainly because MP3 is a delivery format, not a mixing format. Mixing should always be done with PCM files. The only advantage of compression is that you get a smaller file. Disadvantages: it is of lesser quality, and it takes MORE CPU power to play back.
Also, within the bounce window you can navigate ANYWHERE to put your file, though Logic will try and point to its current "Project Folder"/Bounces.
If you still can't find it, just do a Spotlight search. By name, or by creation date, etc.
Also, it is not recommended to bounce to the desktop, there are reports about erratic behaviour when doing that. Just bounce to the Project Folder, it makes more sense in many ways.
The desktop is actually an open folder within your User folder: Macintosh HD/Users/"you"/Desktop unlike any other folder it always shows its content when that user is logged in.