Are these "left over" after a crash?
Probably. I programmed some kind of function that would delete automatically outdated temporary files, but didn't release this feature because I felt that auto-deletion was too dangerous.If some programmers are reading this thread, I would be pleased to share ideas about this feature. See getDefaultTemporaryFolder method in OperatingSystem class and don't hesitate to comment my ideas.
Can I delete them without any problem?
Yes, once you exited Sweet Home 3D and/or its online version.Most have a "prefix" of 2500, the last ones 2600. I guess that's the version number?
Right guess. 
I used this prefix to let me change temporary files management in future versions without preventing users from using older versions.
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Emmanuel Puybaret, Sweet Home 3D creator
