Sometimes the flow isn't enough to clear that bubble. Are there bubbles coming out the returns indicating a continuous injection of air on the suction side? If no, then ignore the bubble if it's small and not causing pumping problems. Air could have gotten in through the skimmer if the flow in the skimmer forms a vortex. Sometimes this will momentarily cause a little air to get in. If it is indeed an air leak in the plumbing it could be so small that water doesn't leak out and may only leak under a slight vacuum. If it is a joint problem then smearing a little silicone seal around the joints while the pump is running should cure it. While running, the silicone will get sucked into the leak before it cures. If you suspect a bigger leak then a stethescope may help using the open tube to sniff the joints but since you don't see water leaking the leak may be too small to hear it.
Hope this helps.
Al
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