Do variables contain extra hidden metadata - aka When is zero not zero (but still is)

Not metadata, just the sign bit of the double precision float.

>> a = 0-j;
>> b = -j;
>> ra = real(a)
ra =
     0
>> rb = real(b)
rb =
     0
>> ra==0
ans =
     1
>> isequal(ra,rb)
ans =
     1

Looks the same so far. However, the difference is that with b, we set the sign bit for both the real and imaginary parts when we do -j = -complex(0,1) vs. 0-j = complex(0,-1) (see Creating Complex Numbers). Looking deeper with typecast, which does no conversion of the underlying data:

>> dec2bin(typecast(ra,'uint64'),64)
ans =
0000000000000000000000000000000000000000000000000000000000000000
>> dec2bin(typecast(rb,'uint64'),64)
ans =
1000000000000000000000000000000000000000000000000000000000000000

That 1 is bit 63 (of 0) in the IEEE 754 double precision floating point representation:

Voila! -0 exists in MATLAB too!