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:
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.