I came across something this morning saying that Census workers can enter your household in your absence with the permission of the landlord.

What many Americans don’t realize, is that census workers — from the head of the Bureau and the Secretary of Commerce (its parent agency) down to the lowliest and newest Census employee — are empowered under federal law to actually demand access to any apartment or any other type of home or room that is rented out, in order to count persons in the abode and for “the collection of statistics.” If the landlord of such apartment or other leased premises refuses to grant the government worker access to your living quarters, whether you are present or not, the landlord can be fined $500.00.

What I find interesting is there's a couple problems here. In Washington at least the Landlord Tenant act says the landlord may not enter the premises without 48 hours advanced notification. Further the census website says the following:

Note that the census taker will never ask to enter your home