While this is not an actual denomination, the concept could easily apply to your ancestor’s worship practices and church attendance. Individuals who felt called to attend church may have attended whichever church was closest and whose practices and beliefs were relatively aligned with their own.
This is more likely to be true for those who lived in rural areas or on the frontier. The nearest congregation of the “right” denomination may have been in the next county and too far away for regular attendance. Your relative may have attended the Baptist church as a child and a Methodist church as an adult–maybe for reasons of geography as much as anything else.
Don’t assume that members of your “long time” Presbyterian family would never have attended a church of another denomination. Geography may have made that decision for them.