Hello:
The conventions for mirrors go as follows. If an object is on the same side of the mirror that the light is coming from, then the object distance is positive. If the object is on the opposite side of the mirror that the light is coming from, then the object distance is negative. I guess it is possible to place an object behind the mirror (i.e., on the opposite side of the reflective surface), it won't do you any good for viewing the image though! But, I suppose it is possible.
Hope this helps,
Robert