A test with a smoke machine running plain distilled water with no glycol. It produced plenty of very short lived steam, but no real visual effect. There may have been residual traces of glycol contaminating the output slightly.
It's not a good idea to store fog machines with plain water in them as the pump requires the lubricating effect of glycol and it can cause it to seize up. (See my other video on pump maintenance.)
Some hire companies flush their machines through with a mixture of water and white vinegar to clean residue from the heater block. This should be done outside as breathing hot vinegar vapour is unpleasant. After cleaning, the machine should be flushed through with standard fog fluid before storage.