Police believe that a small number of people may have left the beach and dispersed following the landing of two rigid inflatable boats containing suspected illegal immigrants in the Hastings area on Thursday morning (3 October).
It is thought they probably crossed the English Channel from France overnight, coming ashore at Hastings, and further east near Winchelsea Beach.
17 migrants, including men, women and children, have been detained by immigration officials, but a few others from the Winchelsea landing are thought to have disappeared into the surrounding countryside.
Detective Inspector Jon Gillings, of Hastings police, said: “Two of those detained required medical treatment and we are concerned that one or two others still outstanding may also be in need of assistance.
“We would ask anyone seeing strangers in the area who may appear cold, wet, hungry, disorientated, or in need of medical aid, to report it by calling us immediately on 999, quoting serial 276 of 03/10.”