The SightingsImport.Log file is created anew on each import. It is created in the directory that contains the file you are importing.
The Open Log button opens the last log file of whatever you imported; e.g. Sightings, Checklist, Locations, Taxonomy, etc.
So, assuming you clicked on the Log File button after importing your BD Mobile sightings, it should ONLY contain messages from that last import. Note: If you selected multiple files to import when prompted, then the log file will contain all messages from all log files imported.
So, with this info, I still need to see your import file and your settings on import.
If you deleted your import file after import, easy enough to call back up that Completed Outing from BD Mobile, Finish Outing, Send and Close and include me in the email address.