IXMIQUILPAN, Hidalgo — On a misty, moonless night, the group scurried down the canyon wall, their feet slipping in the an kle-high mud. The sirens grew louder as their guide, clad in a ski mask and known only as Poncho, urged them to run faster. “Hurry up! The Border Pa trol is coming!” A couple in matching de signer tennis outfits loped awkwardly along, the boyfriend clutching a digi tal video camera and strug gling to keep the pop-out screen steady. The 20 or so people flee ing the Border Patrol aren’t undocumented immi grants — they’re tourists about 700...