Pool Clown,
I believe what you are seeing is the dissolved air in the water outgassing under low pressure. This is very similar to what happens right before water starts to boil or vaporize. At sea level, water will vaporize at 212 deg F. However, air bubbles will form well below this temperature due to air outgassing. The air cannot remain dissolved at higher temperatures and/or lower pressure since solubility depends on both temperature and pressure. However, this is not the same as reaching the vapor pressure of water. So outgassing can occur even if cavitaion does not. With outgassing the bubbles would form with the pump on in the pump basket but disappear (i.e. redissolve) when the pump is turned off. Outgassing is still a condition of high vacuum and high suction head loss but it is not as bad as cavitation.
Again, air bubbles in the pump basket can be caused by several factors and are not an indicator for cavitation. The best way to determine if cavitation is occurring is by the sound of the pump. In a pump, when cavitation does occur, the air bubbles are pushed outwards along the impeller and collapse very quickly so you would never see the bubbles in the pump basket but you will see the outgassing bubbles during cavitation so that is why it is often sited as a indicator for cavitation when in fact it is not. Outgassing can occur without cavitation but cavitation is usually accompanied by outgassing.