What is french siren number?

Answer 1

It's the code used to alert when something goes wrong in a company.

Historically, it was the code that was played with a fire alarm to inform the owner of a company not in the neighborhood of something going wrong at his company, and that he had to come quickly (accident in a mine for instance). The code (actually several) was well known by all employees and written everywhere.

Today this is a code you write on an invoice for instance, when you want to put a curse on somebody. If you receive an invoice with a siren number, you need to forward it to 10 other persons or you are in troubled or ill very soon.

That's incredible they continue to do these kind of things in 2010 in France.