'I know that your mother is in town, since I saw a Rolls Royce parked outside your home.' What kind of fallacy does this indicate?

The claim that the mother is in town rests on a link that is never established.

Seeing a Rolls Royce outside proves nothing about who is present.

The connection between that car and the mother is simply assumed, not shown.

An inference built on an unproven premise cannot yield knowledge.

Reasoning from such an unestablished ground is the fallacy of unestablishment, asiddha.