What is the ruling on water remaining in the mouth after rinsing and then being swallowed by someone who was ignorant of the ruling and cannot enumerate the days this occurred?

After rinsing the mouth and spitting out the water, usually nothing remains in the mouth. However, if one assumes that some water did remain and the fasting person swallowed it intentionally, then his fast is broken and he must make up for it. If he does not know the number of days, he should take precaution and make up what clears his conscience. But if he was ignorant that this breaks the fast, then his fast is not broken and there is no expiation upon him according to some verifiers, but making up the fast is more cautious.