I'm travelling in Europe right now, and in France, Spain, and Italy I've had croissants and similar pastries. Unlike the ones I've had in America, these have a bit of sweetness to them - my intuition is that something like honey or agave nectar has been drizzled on top, either before or after baking. Does anyone know for sure? I'd love to try baking pastries more often when I get back home.
In Rome I was informed that I was eating a croissant with honey on top (after informing my host that I had a honey allergy). It had the sweetness that I suspect you're experiencing. To help you determine if we were trying the same thing: the croissant I had was shiny and slightly sticky on top, and it seemed more like it was brushed on top rather than drizzled. That would support @Carmi's answer that it was brushed on with the egg. It was definitely honey, though, not sugar syrup.
Sometimes, in the less refined areas, you'll get pastries with sugar syrup brushed on top with the egg. It gives a shine to the pastry that lasts for a long time, protects it from losing moisture and makes it sweeter.
This is why all the industrial pastries are usually so sweet.
I don't think that any traditional European patisseries have ever used Agave for anything, and I hesitate to think what their reaction would be if you suggested it.
The only usual topping on a croissant is a brush of egg to give it a golden finish. I suspect the sweetness you are detecting may be due to sweeter-tasting, European butter.