A missing ground conductor will cause incorrect readings on many 'receptacle testers.'

Without a ground there's no way to tell polarity. You have to have a reference.
As brickeye points out, having unterminated connections will cause enough induced voltage to trigger a high impedance meter or light up the little bulbs in these things.

To be sure, find some known ground (perhaps a water pipe) and measure between the small slot and that (should be 120V) as well as between the two slots.