But one evening I made an exciting discovery: balanced just so on a windowsill, my iPhone had snatched a stray sliver of signal and garnered 50 e-mails.

This charge transfer is due to the mechanical release or closure of the contacts, the contact-to-coil capacitance, and the stray capacitance between signal and relay drive lines.

VCM, the common-mode voltage between the sensor and the signal ground, results from nearby noise sources resulting from stray capacitive coupling.