spare bolt / no nut on Focus

Hi, I posted this under another thread but it was a bit hidden..
I have a 99 Ford Focus 1.8 zetec E and just got it back from the garage where it had a 'service' (an oil / filter change) .
When I got it home I checked under the bonnet. At the front, very close to the ground is a big metal strut that runs along the front of the car from one side to the other. It might be supporting the radiator / fan. On this strut was sitting, on its side and unattached to anything , a bolt. !! It was below the drivers side light. There is a hole close by and the bolt will slide into it, but there is no nut present to secure it there ( I suppose it could have come off the car). At any rate, I cant see that there is anything that needs securing. It looks like it could be an earthing point, but there are no wires nearby.
Any clues where this nut might have come from? Does anything need to be removed to get to the oil filter? I just can't see why there would be a bolt on this strut when there is nothing to secure. So it means its from somewhere else which is just worrying...
I am going to take a better look tomorrow when its light.
Thanks,
Steve
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is it a nut or a bolt ?
As you refer to both.
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Its a bolt. It has a thread on it that a nut could go on . But if a nut existed then it has fallen off onto the road.
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Steve R wrote:

It's a standard engineers ploy to drive another engineer mad. You wait until your colleague has reassembled the thing he was fixing then when his back is turned you surreptitiously place a screw where he will find it later. Sit back and watch him rip the thing apart to find where it came from.
--
Malc

You know it's a bad day when even the aubergines are plotting against you
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