Where are they created? Have you tried opening them with a text editor?
You say you tried to delete them? Does that mean you couldn't?
See www.grainge.org for free RoboHelp and Authoring information.

@petergrainge
*.ing is probably actually *.Lng for language file.
*.stp is the stop words file
*.syn is the synonyms file.
in RH2019 New the *.lng file should be called <language_code>.lng (e.g. en_UK.lng) and live under <myprojectfolder>/publish/language/label. Similarly *.syn will be <language_code>.syn under publish/languages/synonyms. And stopwords under publish/languages/stopwords
I have seen some random files created outside the project structure, but I haven't been able to reproduce it reliably. (For my test projects, I'm leaning towards some quirks related to merged projects and links/references *hacked* to work based on output path. But I need to sit down and do some scientific testing...)
If you could successfully delete the files (as per Peter's question) did opening the project recreate them outside the project folder?