The only way to find Iron, Wood and Mandrake if the province has already 40% explored (In total NOT that you have explored 40% MORE) is if there is a special event that happens. This happens rarely ofc but it can happen and does not have any link to how explored the province is to the best of my knowledge.

What the heck level is your Scout todo this and what kind of army does he have?!?!?! Got to wonder if you even need the arena if your guy is that strong.

I’ve never tried it at an arena (repairs and replacements alone would make it hard) nonetheless Parking a scout is a valid strategy and can build your economy up whilst waiting for the enemy to arrive but I’ve only ever done it on my own capital. In essence you scout (battle) the province every turn and use the victory prise to build up the castle as required.

The advantage;
Explores the capital and gives lots of money/ experience which is normally the fastest way to build a scout as long as there are plenty of locations (most capitals work well) then you should be able to deal with the enemy Wizard [or whatever] when they show up… the fact that you don’t go anywhere makes repairing armour and replacement of peons a non-issue.

The disadvantage;
Your economy will collapse over night as soon as you stop so don’t hire expensive guards or you’ll end in negative credits before you can fix it.

IME it works best if you have a Warrior running around capturing provinces but I have also done it with just the scout.

p.s. Don't try it against a human player.

Asked dev:
Common resources, unlike rare ones, don't have strict rule about placing them as deep as possible. They can be anywhere on the %explored. Minimum % of needed exploration decreases as you go on from demesne.