We performed Ru-101 nuclear quadrupole resonance (NQR) measurements on the s-wave superconductor CeRu2 and found oscillatory behavior in the spin-echo amplitude at the vertical bar +/- 1/2 > <-> vertical bar +/- 3/2 > transitions but not at the vertical bar +/- 3/2 > <-> vertical bar +/- 5/2 > transitions. The modulation disappears in the super-conducting state or in a magnetic shield, which implies a geomagnetic field effect. Our results indicate that the NQR spin-echo decay curve at the vertical bar +/- 1/2 > <-> vertical bar +/- 3/2 > transitions is sensitive to a weak magnetic field.