[Imp]

Trying to keep it simple spotting a new unit depends on many factors

1) I think you only want units to see 10 hexes so set the battle visibility to 10 & make sure all units have a visibility of zero

2) The main things that effect units ability to see are
Terrain > better cover equals harder to spot
Unit is moving > the faster a unit moves the worse it sees & the easier it is to see it.
Example A stationary unit sees better & is harder to spot than a moving one so ignoring everything else the stationary unit will always see the moving unit first.

3) Unit size > The bigger a unit the easier it is to spot
The game gives squads size 1
Cavalry size 2
Single men or small groups like scouts size 0

Size zero are very hard to spot so probably dont use them

4) Experience > Higher Exp units see slightly better & are very slightly harder to spot

5) Units that fire also become easier to spot
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I would therefore suggest doing the following
1) If you want all units to be spotted equaly give them all the same size
2) Make sure all units have a visibilty of zero
3) In prefrences turn training off & set troop quality to at least 70, I would try 80 - 100
(The higher the more likely a unit is to spot another)
4) In prefrences adjust searching higher is easier to spot, adjust this till you get the results you want.
I would suggest setting higher say 120-130% at a guess as units will not fire so its harder to see them.

As a thought if you want units to be seen easily give them a bigger size, size 3 plus units will be seen quite easily.