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Detect magic in 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?

Please cite or at least clearly reference all rule text you use to get to an answer.

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No, because the aura is only around magical objects.

The spell states,

If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

It does not say that there is any "background" magic to all mundane items. That is, you only ever get a positive result (magic is here), and never a negative one (there is a distinct absence of magic here). For example, you could cast the spell when there is no magic around, and you would not detect anything because there are no auras to be seen. Therefore, if an item is blocked by one of those barriers, you would simply not see any aura.

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    \$\begingroup\$ This answer might be improved by describing what would happen if you slowly moved a magical weapon behind a sheet of lead, compared to doing so behind a sheet of steel, as I believe this would illustrate a way to actually detect a thin sheet of lead with the spell in question. \$\endgroup\$ – Yakk Jul 19 '17 at 19:46
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    \$\begingroup\$ @Yakk That sound much less like an amendment to an existing "no" answer and much more like a separate "yes" answer you may want to submit. \$\endgroup\$ – SevenSidedDie Jul 20 '17 at 2:38
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No, because…

Spells do only what they say they do

Being blocked by lead isn't something that detect magic's description says it will report to the caster, therefore it does not report the fact to the caster.

Detect magic does say that the spell does not reach beyond an appropriate piece of led, so it does what it says: nothing at all.

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

The spell itself is non-functional beyond such barriers, and its description includes no “detect barrier” function even within its area of effect.

The impossible citation

Sadly, there are no rules written in D&D 5e that say that the absence of rules as written to create an unusual effect does, in fact, mean that no such effect is created. So there's no possible in-rules citation to that pragmatic truth within the text, and only our shared understanding of how communication works can be cited.

The fact that spells only do what they say they do is also sadly not citable in the rules themselves, as it's a basic principle of writing and reading statements that dates back beyond its own documentation. As a principle it was well-recognised enough to name it, by readers of William of Ockham (1287–1347), who applied this fact of reasoning to various subjects in his works, and posthumously named the principle after him as Occam's razor. (It's so named because, like a razor, it cuts away gratuitous suppositions and facts that are unnecessary to establish a correct understanding of something. In this case, Occam's razor cuts away any extraneous need for the text of D&D 5e to include instructions that it should be read in the usual way that instructions are read.)

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