'My mother must be visiting, since there is a Mercedes parked outside.' What kind of fallacy does this indicate?

A Mercedes parked outside is too general a sign to fix on the mother's visit.

Many people own or drive a Mercedes, so the reason points in many directions.

The same evidence would equally support quite different conclusions.

A reason that wanders like this fails to establish one definite result.

An inconclusive, straying reason is the fallacy of deviation, savyabhichara.