I found this on a radiology site:
"There are two potential spaces between the leaflets of the septum pellucidum in adults. The cavum septum pellucidum (CSP), which is anterior, is separated from the more posterior cavum vergae by the anterior columns of the fornix. During development, these spaces obliterate postero-anteriorly: the cavum vergae followed by the cavum septum pellucidum.
Cavum septum pellucidum is a normal variant. Because it is filled with CSF, CSP is also sometimes referred to as the fifth ventricle; however, the most common type of CSP is not connected to the ventricular system, and the use of this term to refer to CSP should be discouraged. "
So, if I am reading the medspeak correctly, it can be a normal variation - and if you read up further, it may have been there since birth. Since MRIs take different slices each time, and it is tiny, it may have been there, just fallen between the slices.
Hope that helps until you see the doctor and he/she really can tell you - and the rest of the report is okay.