I think (a) is correct. Am I right?
(a) I received a faxed document yesterday. There was no fax cover page so I am not sure who sent it
(b) I received a faxed document yesterday. There is no fax cover page so I am not sure who sent it.
(a) is far more likely, but (b) is possible, especially if you are holding the document in your hand as you speak.
Context is always important; labelling is rarely important.