I think my Filly has Rabicano?

yes I realize that but she has more than a few white hairs on her flank. Her tail, however, has a few white hairs. There is definite roaning on her sides.

Roan/rabicano can present minimally, but since you say she has no roan or rabicano in her lineage, it seems unlikely that she has it. It could be random white ticking, or it could be caused by sabino (although she doesn't seem to have any white markings, so maybe that rules that out?)