Most concrete I swing over has metal in it but there have been some, like some older sidewalks in my neighborhood,where it stays silent.
BTW, you mentioned dishing out iron and your detector still going crazy...and you were surprised.
Let me clear up something that will help you out for the rest of your time in this hobby.
When you use disc that doesn't mean you are necessarily shutting out metals.
Many things, like the big metal that is in this slab, can still bleed right through that disc no matter what you do.
In this case when you told your detector to knock out iron all you did was tell it you didn't want to see, hear or know about it...but it still could.
Your tool could still sense it just the same and be affected by it.
I believe the same thing happens out in the field with much smaller problem metals like nails near coins to a lesser extent.
Yes, some tools can disc out the nail and still hit pretty good on the coin as when using Monte's nail board test, some better than others, but even the best ones are still affected by all metals in the vicinity.
Even using and listening to my Tesoros or even my Fishers on a target like a coin next to masking trash like a nail I can hear a good enough and solid enough tone to get me to dig it but take that nail away and listen again...
The tone over just the coin usually gets better, sharper, a bit more solid...different without that other metal affecting everything.
That is no matter how much disc I am or I am not using.
Discrimination is not magic, it is a bit of programming and tech that helps us cut down on the sounds, signals and noise levels of unwanted areas of conductivity so that our ears and brain will not be confused by tons of signals and helps us pick out, hear and see the more wanted signals of the better targets we go after.
Should it be trusted all the time?
That depends, I found in some areas and site conditions it works well, in others like sites with a lot of iron, sometimes heavy trash and in mineralized soil disc can hurt you and works to cut out a lot of the good targets along with the trash or won't let a good enough signal on good targets come through so we can notice them...sometimes way more than you think.
That is why in the areas I hunt now which are mineralized and filled with a crazily amount of extra helpings of trash and iron I found when I use less disc, either all metal or my disc setting on 0 or at the most 1, I can notice and find more.
I had to learn to use settings like these over time with practice, it wasn't easy, very jumpy, noisy and confusing but it is doable and just something that I needed to do to stay the most successful.
If I hunted in beautiful, clean, black low mineralized dirt things might be different but they are not...so I learned to deal with it the best way I know how.
