I finally found out what this is. And it's a bit of an anti-climax.
My parents have an ancient paper encyclopaedia. And under 'beetles', there it is. Apparently, it's so common, that it features in the only picture accompanying the beetle-entry, comprising about 15 drawings of beetle species (considering that 30-40% of all insects are beetles, that's not a lot). Now that I know what it is, I have googled it. It is said to be so wide-spread in European gardens, that I can't believe no one I showed it to has ever seen one before! It's not like it's tiny to the naked eye or something. The encyclopaedia is rather exact about it; it measures 2.9 cm. But hey, that was back in 1975 (as I said, the encyclopaedia is ancient). I'm pretty sure this one - and the one that passed through Tuesday a week ago, because once you see one, apparently, you see all! - was much bigger. At least, and I say at least, 3.1 cm.
It's a devil's coach horse beetle. Which does juice up my imagination. More so than its Dutch name, which is, literally translated, the smelly short-shield beetle. It's not poisonous, but its bite can be painful. Poor Baby Boy...
& did he learn a valuable lesson? No, he didn't. This is him trying to lift the glass off last week's visitor (before you worry, my hand is firmly on top of the glass, taking the picture with the other hand, doing what a mother does best: multitasking!).
Anyway. Mystery solved!