'Series of bends' and 'an S bend' is IIUC now the same sign (used to be slightly different) in UK. In Wales at least I just treat that as the default on pretty much all roads, including parts of The Motorway
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'Cattle' usually means there is a cattle crossing nearby - spot the gates they come through and the ingrained mud on the road. There are many unsigned areas where farmers take their animals across or along roads.
'Deer' simlarly is a sign used where they herd nearby but people may not expect deer on the road; I've lived and driven in plenty of places where they abound and have seen far more where there are no signs (interesting to have one canter alongside trying to cross ahead - if you slow down you just know it will cut directly in front!).
'Slippery surface' you should be able to make out the conditions which lead to this sign being placed - usually under trees, on a slope where water crosses or in a dip where ice forms - sometimes replaced by the 'ice' sign, though that IME is more used on exposed longer stretches (not seen anything official about that, just my impression).
'Horse riders' is frequently seen without a sign of a riding stables or horses, yet around here with occasionally plenty of horses on the roads, is not something often used.
Of course these are all placed where a Highway Engineering team (or a road worker) decides to place them, potentially the conditions change (slippery roads get repaired with bettter surfaces, trees are cut down) and the sign remains. As you say, they should require little
extra attention from your normal drive because any of these can occur without signage.
'Mobile roadworks' is particularly one that would get my attention. It can be left out by accident, but since the roadworks are
mobile, they may have moved some way beyond the sign; so I keep expecting them until they appear (or it is very clear that they are not there - e.g. I have left that road completely).