This is Place Antoine Repaire, almost the northernmost point of Monaco at the border to Roquebrune and Saint-Roman in France. The stone monument here is in the middle of the picture.  There is a Monegasque police station in the left photo part. This image is photographed from southeast towards northwest.  All available maps confirm that the photographer is standing in France.

Same motif as above, but zoomed.

From east towards west.  According to some maps the photographer might now be inside Monaco.

Slightly closer.  

Most police officers in Monaco are French citizens.  (The Prime Minister of Monaco is usually also a French citizen.)  We asked this policeman if he could tell where the boundary goes, but he answered very politely that he could not.

A welcome menhir.

Some articles suggests that the monument in the middle of this interchange is located exactly on the French - Monegasque boundary. Photographed towards south and the ocean.

The same monument from the opposite side and with the menhir in the background.

According to the tourist maps the Monaco monument is about 10 metres inside Monaco.