Having owned 3 406's, they have all had various conectors, just like yours, hanging down with no obvious place to go. If you like, perhaps tydy it up with a zip tie? Or just dont worry about it. I presume they are for extra stuff thats not fitted.
1999 HDI 110 GLX Estate Sold On at 230,000 miles to the lucky John
2003 HDI 110 Rapier Estate
1998 D8 1.9XUD Estate LX 7 seater Estate sold, with regret
1999 306 1.8 petrol.
Hard to guess by the picture,
but from here it looks like a dummy plug, as said above, you will find others too,
that may look like they shouid be plugged into something but there not,
they dont make alterations too looms, so some of the extra plugs might be required but on diffent ranges of engine,
if everything works on the car, iwouidnt worry about it, just cable tie it out the way