I imported all photos in a file on my hard drive, but several photos I didn't recognize (and were nowhere on my computer) showed up in the import. This has never happened before and I would like to know if Lightroom somehow grabbed the items from the internet or some other source.
If the photos don't show as missing then you can right-click one of them and select "show in finder" on the Mac or what ever the equivalent command is in Windows. This will take you to where the photo is on the drive.
LR doesn't grab images from the internet or some random sources. It only imports images from the drive, folder and subfolders or from the memory card from your camera that you point it to in the Source section on the left hand side of that dialog window. If you have Include Subfolders checked in the Source area then any and all subfolders under the drive or folder you have selected will be scanned and whatever images are found will show up.
So if select my C: drive as the source and include subfolders this is what I end up with. It's scanning my total C: drive for any image file.
It found over 5000 images.