Do the material(s) have a long text, e.g. purchase order text, sales order text, basic data text?
If yes then you have already entries in another table -> in STXH
does your material have plant view? then you have an entry in MARC,
Does it have valuation data?
Just go to DB15 and lookup the tables for object MM_MATNR and you will see how many tables could potentially have an entry for a material. And this only if it really was not used anywhere else.