The iNode file is what is left over after the fscheck (disk repair) does its job. It is usually an orphaned block of data (inode) that was not properly dereferenced when a file was deleted. Inodes in general are low level data structures, however, a file named iNode is no more low-level than any other file. The disk repair utility finds these orphaned data chunks, dereferences them, but saves a copy in the lost+found just in case they contain important data and the used had tried to brute-force cancel an unintended delete. If not needed, these files can be safely deleted.
In this case, Finder or the AppStore app had most likely crashed or had to be force quit while in the process of deleting the file after the successful OS upgrade.
In my case my iNode file has 4172M, and is an archived volume named "Mac OS X Install ESD", containing
the folders "Install OS X Mountain Lion.app", "Library", "Packages", "System" plus a whole bunch of hidden files.
You are right that the mystery file has nothing to do with MS Excel.
Click on the search icon on the OS X menu bar (the magnifying glas at the right), type "terminal" without the quotes. Click on the application called Terminal. Once the Terminal window opens, type the following: "man xar", without the quotes, then press enter. You will be presented with the system manual for the archiver called xar. Use the up/down arrows to scroll, or press "q" to exit.
Reading through that documentation it should become obvious that xar is a file archive/compression format similar to the popular ZIP. The unknown file is the downloaded OS X upgrade packed in the xar format.
The XAR project aims to provide an easily extensible archive format.
Important design decisions include an easily extensible XML table of
contents (TOC) for random access to archived files, storing the TOC at
the beginning of the archive to allow for efficient handling of
streamed archives, the ability to handle files of arbitrarily large
sizes, the ability to choose independent encodings for individual files
in the archive, the ability to store checksums for individual files in
both compressed and uncompressed form, and the ability to query the ta-
ble of content's rich meta-data."
My iNode6689314 file is 5,29GB and created 23 of Oct this year. That aught to be when I installed Mavericks.
On macrumors Bernuli posts that
What you see in lost+found is the result of a filesystem check, or Disk First Aid running and finding a some things out of alignment. Not a big deal, unless you have a bazillion of those files. Then you should find out what is causing it.
That seems likely in my case.