Are strategic resources secretively revealed before the required technology?
Solution 1:
To answer in short, 'future/undiscovered' strategic resources are not revealed earlier by bonuses.
Industrial Zones receive a:
- (+ 1) Production yield for each adjacent mine or quarry.
- (+ 1/2) Production yield for each adjacent district tile (this "Minor Bonus" rounds down).
As of Gathering Storm, this changes to:
- (+ 1/2) bonus for each adjacent district, mine or lumber mill (again rounding down).
- (+ 1) yield for each adjacent strategic resource, government plaza, and quarry.
- (+ 2) for each adjacent aqueduct, dam, canal or bath.
Civ 6 Resource Wiki Page
The reason you may be seeing that '+ 1 for strategic resource' is because your commercial hub is actually providing you an unseen strategic resource. If you have only discovered niter, it's likely that or iron or some other early resource. Hover over the commercial hub and you can see if there is a hidden resource underneath.
You can place district, wonders, and cities on tiles that contain strategic resources, even unrevealed ones, and upon discovering the resource, you will gain use of it as if it was improved. Note, this only applies to those tile enhancements. Ex: if you have a farm and discover niter underneath, you need to remove it and replace it with the appropriate improvement.