That is nothing but an assumption. The Paths have to undetectable too. When Shima felt their presence in Konoha, it was her intuition and gut feeling, not her chakra sensing ability, otherwise Jiraiya would still be alive with both his arms intact.Its pretty simple actually since they had just killed the paths and did not know there were six and that one could revive the others they let their guard down because they thought they had won.