Thank you and nosebear for the clarification! Now I understand better why some of my colleagues (like my boss, older) use "Du" and some don't. I'll stick to not using it, there are enough grammatic pitfalls elsewhere in the German language (not that French is any easier for foreigners, I'm sure).
Interesting how you write "Du" and not "du". I'm French and I've been living in Germany for 20 years. I understand and use "du/Sie" more or less appropriately (we have the same dichotomy in French). What I still cannot wrap my head around after all this time is why/when some people use "du/Du", "dein/Dein", "dich/Dich" in writing (to be clear: not at the beginning of a sentence). I guess "Du" is somewhere inbetween "du" und "Sie" on the politeness scala but I never dared to ask. I'm only using lowercase "du". What would be a rule of thumb on how/when to use the uppercase "Du"?