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 I programmed some kind of function that would delete automatically outdated temporary files, but didn't release this feature because I felt that auto-deletion was too dangerous.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.