The framed oil paintings were found by a 64-year-old man at a motorway service station in the Bavaria region last month. The man then turned the paintings over to the police in the western city of Cologne, the police department said.
Agents have appealed for the owner of the paintings. An initial assessment by an art expert concluded that the paintings were likely original works, police said.
One of the paintings is a smiling self-portrait by Italian artist Pietro Bellotti, dating from 1665.
Bellotti is best known for painting portraits. According to the Galleria Canesso in Switzerland, the artist “worked for high profile families in Venice and beyond,” including patrons such as Cardinal Ottoboni and the Governor of Milan.
Hoogstraten was a painter and writer trained under Rembrandt in Amsterdam, according to the Leiden Collection, one of the world’s largest private collections of works from the Dutch Golden Age.
At the end of the 17th century, The Hague’s elite “lined up to sit” for the portraits of Hoogstraten, the Collection said.
The artist also wrote an “Introduction to the Haute Ecole d’Art de la Peinture”, which was published the year of his death, in 1678.
It includes reminiscences of his stay in Rembrandt’s studio and is what the National Gallery of the United Kingdom has called “a valuable source of information on Rembrandt’s views on painting”.
Elle Pickston of CNN contributed to this report.