It could be the rectifier has partially blown, this converts the ac current the alternator produces into dc current. There are four diodes in it. If one blows then it will only convert half of the ac current to dc. This can cause the warning light to come on at low revs. I discounted this as you said the alternator was recently reconditioned but it is possible. You wouldn't be able to tell unless you have access to an oscilloscope to check the voltage waveform though.