Electrical – this low-voltage thing on the attic light

I was not able to find any sections in NEC that prohibit switches from being installed in closets, so until somebody can point to a specific section I'd say it's not a problem. There may be adaptations to the code in your local area, so you'll have to check with the local Electrical Inspector to be absolutely sure.

The NEC does, however, have a section on luminaires in clothes closets.

NEC 2011

410.16 Luminaires in Clothes Closets.

(A) Luminaire Types Permitted. Listed luminaires of the following types shall be permitted to be installed in a closet:

(1) A surface-mounted or recessed incandescent luminaire with a completely enclosed lamp

(2) A surface-mounted or recessed fluorescent luminaire

(3) Surface-mounted fluorescent or LED luminaires identified as suitable for installation within the storage area

(B) Luminaire Types Not Permitted. Incandescent luminaires with open or partially enclosed lamps and pendant luminaires or lampholders shall not be permitted.

(C) Location. The minimum clearance between luminaires installed in clothes closets and the nearest point of a storage space shall be as follows:

(1) 300 mm (12 in.) for surface-mounted incandescent or LED luminaires with a completely enclosed light source installed on the wall above the door or on the ceiling

(2) 150 mm (6 in.) for surface-mounted fluorescent luminaires installed on the wall above the door or on the ceiling

(3) 150 mm (6 in.) for recessed incandescent or LED luminaires with a completely enclosed light source installed in the wall or the ceiling

(4) 150 mm (6 in.) for recessed fluorescent luminaires installed in the wall or the ceiling

(5) Surface-mounted fluorescent or LED luminaires shall be permitted to be installed within the storage space where identified for this use.

What you are describing is called a Multi-wire Branch Circuit, and it is completely legal (given that it's done properly).

The first problem, is that it doesn't should like the breakers feeding this circuit are handle tied together. It should be feed by a double pole breaker, or the two breakers handles should be tied together. You should not be able to shut off only half the circuit (red only or black only).

As for the voltage. I'd suspect it's phantom voltage (inductive coupling), and so should not cause any problems (especially if the breakers were properly tied together).

If the breaker for the "red" conductor is off, and current is flowing on the "black" conductor. Because the conductors are parallel, and in close proximity, a voltage may be induced on the "red" wire. There should be no current, however. So if a load was introduced, the voltage would drop to 0.

You can determine if it really is phantom voltage, simply by using a low impedance voltmeter. The low impedance meter will read 0 volts, while a high impedance meter will read the phantom voltage. An analog meter is one example of a low impedance meter, though some digital multimeters are able to operate in both modes.