Seek a (hidden) .gsf-save-* file at the same path.
From a run of truss gnumeric, for a file that I chose to save at /tmp:
…
readlink("/tmp/Book1.gnumeric",0xdf13001d200,256) ERR#2 'No such file or directory'
fstatat(AT_FDCWD,"/tmp/Book1.gnumeric",0x820fe4420,0x0) ERR#2 'No such file or...