Four unknown men broke into jailed businessman Anastasios Pallis's war museum in Vari, south of Athens, late Wednesday, according to reports.
Reports said the robbers tied up the security guard before breaking into four containers at the site. It is not known what may have been stolen.
The security guard reportedly called the police after he managed to free himself early Thursday.
Pallis had been arrested in connection with money laundering charges and faced additional charges of illegal possession of weapons.