The photos from or outside Lightroom can go missing only if you delete them manually.
You should check if there is any other catalog in which you imported those images.
Go to File menu in Lightroom> Open Recent> look for other catalogs here and open them one by one to check if they contain those images.
Did you by any chance import using the MOVE option?
Have you used your operating system's search feature to search for these photos by filename on ALL of your hard drives?
Please give a detailed description of what steps you took... thanks.
I agree with the post that said you used the Move option and they got moved to your internal HDD. Are they displayed in the library module of LR? If they are you haven't lost them. Right click on one and select Show in Finder or Windows/File Explorer to see exactly where they are.