Page "Mariner 2" Paragraph 45

The magnetometer detected a persistent interplanetary magnetic field varying between 2 γ and 10 γ, which agrees with prior Pioneer 5 observations from 1960.

This also means that interplanetary space is rarely empty or field free.

The magnetometer could detect changes of about 4 γ on any of the axes, but no trends above 10 γ were detected near Venus, nor were fluctuations seen like those that appear at Earth's magnetospheric termination.

This means Mariner 2 found no detectable magnetic field near Venus, although that didn't necessarily mean Venus had none.

However, if Venus had a magnetic field, it would have to be at least smaller than 1 / 10 the magnetic field of the Earth.

In 1980, the Pioneer Venus Orbiter indeed showed that Venus has a small weak magnetic field.