Is this a common problem...

My mechanic has my car at the moment, at the weekend I noticed my water temp going above 90c which it never does, and also the viscous fan didnt come on,
which caused it to overheat.
It didnt actually overheat properly as i turned it off when I noticed the temp going high, acording to the mechanic a pipe flange has popped out, it connects to a long piece of cast alluminum that streches the width of the engine at the front, behind the cam belt and serpentine belts.
The actual piece is a connector for a rubber hose on the left hand side of the engine, and it has just popped out, the audi dealer's have told him this is a fairly common occurance and you can replace the piece that has fallen out with a new piece, its cylindrical in shape.
My question is, do you think this has popped out due to excess pressure, or is it just a weak point that after a hot summer has decided to give???
The reason the fan may not have come in, maybe is this because the water had escaped from this leaking part, and with no water in the system the fan would not engage as there was no water for it to sense?
Does anyone know how Viscous fans "sense" that an engine is hot and engage its clutch, does it use engine/water/oil temp??
Any help appreciated
A4 1997 2.6 Quattro 12v
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