1. A method for estimating whether an item is in a zone, the method comprising: in at least one initial trial: transmitting at least a first RF wave into the zone; querying, by modulating the first RF wave, the item in the zone; receiving an identifier from the item; and determining, based on the at least one received identifier, an observability of the item in the zone; and in at least one first subsequent trial: transmitting at least a second RF wave into the zone; querying, by modulating the second RF wave, the item in the zone; and upon not receiving the identifier, estimating whether the item is still in the zone based on the observability and an observability threshold.