Marie,
From what you say I think I would just live with it if everything else looks normal. The reason removing the basket makes the bubble disappear is probably the flow pattern inside the basket chamber changes and the air is able to escape with the flow of water. If you're not seeing bubbles at the return jets then no new air is leaking in and that bubble is just the initial air that got trapped and isn't making it to the impeller. You may try switching the filter valve to recirculate for a couple minutes to reduce the pressure and increase the flow a little and see if that lets it escape. Otherwise....don't look at it. I can always make a service call next trip south too.

Al