I just tested some warmish tap water. 1.004.
I guess I missed my FG by quite a bit less than I thought. Although this also means I missed my OG by that much more as well, so I suppose my ABV is still gonna be mid 5s.
Yes, your hydrometer is definitely off, but "warmish" isn't good enough. You also need to know the exact temp of the water, and preferably adjust it to the calibration temp of the hydrometer, probably 60 degrees. It will be indicated somewhere on the tube.