I posted a few months ago about this. The highlighted bolt was very loose so I tightened it back up, albeit by hand, and thought nothing of it. So today I checked it and: The bolt has gone!
Sorry! It's a '97 Golf Mk3 TDi with engine code AHU. It's the front of the engine, to the left and slightly in front of the sump pan and is joined to a belt.
looks like the PAS pump bracket. The bolts are available from VAG still, and they're pennies (well about 300 of them for 2)