In sentences like this, where the subject may be unknown which noun class prefix should be used?
Since "Ki-Vi" is the most common noun class, I would think that you would use that class. Kenyan Kisahili may differ a little because we use a "neutral class" when the subject is unknown. The class is defined as "I-Zi", so "kinaenda" would become "inaenda" (singular) or "zinaenda" (plural).
Both ki/vi and i/zi are used in Tanzania, unless you are talking about people/animals, where a/wa are used.
Normally i/zi would be used for an unknown subject, but it's tricky because if it's an animate unknown, it would be a/wa. In this case, it might be implying kitu (thing), but I'm not sure.