If a unit passes through a place that has been reserved for construction, that unit will be visible.
If you reserve houses, fences, etc. in the enemy’s territory, the enemy will be visible.
Since the enemy does not know that you have made a reservation, you can unilaterally see the enemy’s situation.
I want you to do something
I’ve heard of scouting with sheeps, but scouting with constructions ? That’s crazy enough that it might work.
Once you place the building, the enemy can see it and destroy it in one hit. I don’t know if the presence outside of your line of sight blocks the placement of the construction zone, but if so you’ll only know the enemy has something, be it a single villager passing through at the moment or an entire army it’s impossible to tell.
At most for a wonder it’s a 5x5 area, I don’t think it’s worth the effort to barely get any information on such a small area, that may no longer be relevant anyway by the time you get there (units can move).
Are you playing aoe2de or an older version?
As far as i know in de that Bug is fixed
If you reserve a building in an enemy area, the enemy will not be able to see the building, but if an enemy unit passes through that area, you will be able to see the unit that passed through.
Since the building state is 0%, the enemy cannot destroy it, and they are not aware that it is reserved for construction.
I tried it just now, but now it seems to have been fixed and out of sight.
Sorry for the misunderstanding.