Scientists think the leopard could have come to Delhi from Kalesar in Haryana, 400 km from the city, which has a healthy population of leopards.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/delhi-scientists-leopard-spotting-brings-hope
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/leopard
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/scientists
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ecosystems
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lion
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/wildlife
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/haryana