Well, after a few headaches, I found that this should do the trick in single player mode :
function find_lost_character()
surf = game.get_surface(1)
for ch in surf.get_chunks() do
a = {{ch.x * 32, ch.y * 32}, {ch.x * 32 + 32, ch.y * 32 + 32}}
for k, entity in pairs(surf.find_entities_filtered ...