I'm pretty sure this is a flare effect of the filter relecting back into the lens. If you must use a filter (you've mentioned protection against small children and flying goop in another thread. . .:-) ), then you'll probably be well served by looking into getting the best multi-coated filter you can find from a top brand company. Of course, PP can be an alternative, but flare can also cause AF performance problems, or the "ghost" reflections might obscure important details in another situation.
I'd opt fot the MC filter in your case, but it'll probably be a bit pricey -- so then you'll be worried about your expensive filter. . .