A notebook containing 32 sketches by Pablo Picasso, valued at about €8m (£7m), vanished yesterday from the Picasso Museum in central Paris.

The book, displayed in a glass presentation cabinet on the first floor of the 17th-century building, was found to be missing just before lunchtime. There was no sign of a break-in and the museum's alarm system did not go off.

The Ministry of Culture said the cabinet holding the notebook could only be opened with a "special tool" but last night a museum employee said that the cabinet was open because the lock was broken.

Download the new Independent Premium app Sharing the full story, not just the headlines

Police were said to be working on the theory of a planned, or opportunistic, theft during a special opening of the museum to neighbours in the Marais area of the third arrondissement of the French capital. The museum, housing Picasso works donated to the French state by his heirs since 1979, is normally closed on Tuesdays.

The notebook was last seen when the Musée National Picasso, on the Rue Thorigny, closed on Monday evening. Since 2006, the museum has been under renovation intended to double its display area. It is currently hosting a special exhibition of the works of Daniel Buren, which includes a giant mirror snaking from room to room.

The French media speculated last night that the presence of this mirror, by blocking the view of museum staff, may have helped the theft. The French news agency, Agence France-Press, quoted police sources as saying that the museum's security was notoriously "feeble" and that improvements were scheduled as part of the renovation work.

The museum, in an aristocratic town house called the Hôtel Salé, normally contains hundreds of paintings, sculptures, drawings, engravings and ceramics by the Spanish-born master of modern art (1881-1973). Many of the major works are currently on loan, or hire, to special exhibitions all over the world to raise money for the renovation work. They have been replaced by special exhibitions of the works of other artists and displays of minor Picasso sketches and artefacts that are not usually shown. The missing notebook is believed, however, to be part of the museum's permanent display.

There has been a long catalogue of Picasso thefts in France, and elsewhere, in recent years. In March 2007, two paintings worth €50m were stolen from the home of the painter's granddaughter in the heart of the government quarter of Paris. They were recovered five months later.