During the priming process, the pump needs to push the air out of the pump basket into the return line. If there is a suction leak, it will continuously draw air into the pump basket and out the returns. So the continuous air out of the returns is a good indication of an air leak.

You are correct though in that it the pump can operate properly with a small amount of air. But if there gets to be too much air, it can cause pumping issues. A short run at higher speed should take care of that.