Activists placed the cross at the Kurapaty memorial on November 29.
The memorial marks the area near Minsk where at least 30,000 people were murdered and buried by Soviet authorities in the 1930s and '40s.
The chief of the Belarusian branch of the Memorial human rights group, Uladzimer Ramanouski, told RFE/RL on December 3 that police are investigating the disappearance.
Last month, supporters of the pro-communist organization Avant-Garde of the Red Youth reportedly vandalized the memorial site.
The vandalism took place on the eve of the 95th anniversary of the revolution in Russia that brought Vladimir Lenin's Bolshevik Party to power in 1917.