I forgot to say that my version of GNU Octave is 4.0.0

The problem is with the regexp used in
octave-eldoc-function-signatures.  It matches the signatures of the
function, that are output with a call to the GNU Octave function
print_usage.  It also matches an old-style of outputting these function
signatures, which was:
usage:  bar = foo ()

This output was also generated by calling the GNU Octave function usage,
like this:
usage ("bar = foo ()")
But usage has been deprecated in GNU Octave, I think in major version 4.
It's still present in my version, but I don't know about latest releases
(there's a recent GNU Octave 5.1.0)

I don't know why the author chose print_usage over the GNU Octave
function help, but when print_usage fails, due to being called with a
nonexistent function in his argument, like this:
print_usage ('A')
An error message like this one is output:
error: print_usage: 'A' not found

The regexp used is wrongly matching this line and therefore
octave-eldoc-function-signatures is not returning nil, as it should in
this case.

Best regards,
Mauro.