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Detect Magic in 3.5e can be blocked by
1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.
Does a caster know that the detect magic spell is blocked? Is there a noticeable obstacle or signal hole?
The rule text says it can detect
Presence or absence of magical auras.
Please cite or at least clearly reference all rule text you use to get to an answer.
Most of the time, there's no background ambient magic to compare the results of detect magic to. However, at least in 3.5, sometimes there is such background radiation, and a lead sheet (or other barrier) would generate a discernable hole.
For example, detect magic even states:
Lingering Aura: A magical aura lingers after its original source dissipates (in the case of a spell) or is destroyed (in the case of a magic item). If detect magic is cast and directed at such a location, the spell indicates an aura strength of dim (even weaker than a faint aura). How long the aura lingers at this dim level depends on its original power:
And then a list of aura strengths and times is given, with the maximum being 1d6 days for destroyed magic items with CLs higher than 20 (usually artifacts, epic equipment, or custom PC-made stuff) or spells of 10th level or higher (usually high-level spells affected by free metamagic).
Many spells pervasively affect an area, even through barriers. For example, "A cylinder-shaped spell ignores any obstructions within its area". While such a spell is active, and for an amount of time afterward dependent on the spell's spell-level, the region its effects filled will register as magic to detect magic, and thus such suspicious 'dead zones' will be noticeable. Even in such cases, however, the caster does not know without additional work why the dead zone is present-- that is, what is effecting it. It could be a thin sheet of lead or the far side of several feet of dirt or a foot of stone. Furthermore, it may not be an artifact of the metric system used at all and, rather, accurately represent a hole in the original spell (due to, for example, an antimagic field or special properties of the specific cylinder spell used).
There's a lot more than just cylinder spells that can provide such background noise to measure against-- many terrain-like setting elements like mana storms and primal magic events and wild magic stuff and ancient partially-active magitech ruins also might provide you with a source of measurable background radiation, but cylinder spells are notable because you can create them. For example, the second-level Sorcerer/Wizard spell Aerial Alarm from Heroes of Battle has a 500ft tall 100-foot radius cylinder as its area and, while using the spell is difficult since it has only Medium range and thus usually mostly extends underground, this makes the spell useful for providing relatively cheap background scatter at higher levels.
Impede Sun's Brilliance, from Sandstorm, is a 1st level Druid spell also available to Clerics of the Summer domain that also possesses a cylindrical area, but its area of effect is much much smaller so its utility as a Detect Magic aid is somewhat less.
You've already quoted the rule. This is more a matter of what the rule does not say.
The caster is aware of the presence or absence of magical auras. What that absence is caused by, something blocking a magical aura from the spell, like lead or stone or dirt, or the actual absence of a magical aura, is not not mentioned in the spell as detectable. However, that doesn't mean the presence of a blocking material could not be detected by other means; testing whether a known magic item can be detected through the item in question, for example.
This I've always felt is more a character perceptional, and intelligence/experience thing and can highlight the more expansive duel class skill sets and Character professions. And as above regarding detect magic, Lingering Aura it creates an aspect of rules that can be developed or seen by different DM's in different ways.
So when I had similar experience with a Duel class Wizard/Rogue who played in his terms a professional Tomb raider they were from a DM point of view better experienced to be given a little flexibility in how the broader terms of the spell and spell terms are argued and interpreted.
Why because such a character has experience to fall back on. So in a world with high magic, and for example in an old temple ruins, then as a GM it would be fair to say there is background magic to the whole place, just as a wizards workshop would be, its everyday absorption so if they cast the Spell and an area of wall or floor is showing fainter background magic or more noticiable blocks where its suddenly not glowing then that could mean a very think area that just is not magical or something from 'their perception' of the Spells effects has caught their attention.
However such a spell from a town wizard or from a spell stored item, then the description would suggest just the large flat blank areas.
But you may also get a spell caster who is able to cast the spell and then move to a position where they can see the angle of such a blank spot to determine if it is thick or not such as close to a corner or with a pillar etc.
My personal interpretation of wizards seeing magic as like thermographic images with high magic being white/yellow and non magic very dark blue/purple. I.e https://testoltd.files.wordpress.com/2015/11/blocked-radiator-2.jpg?w=604
