Thanks for looking into this.

I find pertinent MATLAB throwing and error in the absence of a variable (rather than a warning) but I actually filled in a bug report here because I didn't want anything to be displayed if an empty variable name is given - adding an ID will fix it for me but maybe MATLAB behaviour could be used here in this special case so as an empty variable name is silently ignored.

There is a small difference in behaviour between Octave and MATLAB when using 'save' with empty variable names or non-existing variables:

In MATLAB:

>> clear
>> a = 1;
>> save('file.mat','a','b')
Error using save
Variable 'b' not found.
>> save('file.mat','a','')
>>

In Octave:

octave> clear
octave> a = 1;
octave> save('file.mat','a','b')
warning: save: no such variable `b'
octave> save('file.mat','a','')
warning: save: no such variable `'
octave>

I.e. Octave returns a warning while MATLAB returns an error for unknown variables.
Furthermore MATLAB ignores silently an empty variable name while Octave issues a warning.

I would advocate for a silent execution for the latter as it would allow the following:

opt = '-mat'; % or '' for no option
save('file.mat','a',opt);