maybe DCF simply went to the wrong house? Maybe person that complained was complaining about the apt. people and they gave DCF addy of the intersection where your road meets main road, and DCF went down the road to your house?
Or maybe young child was out by itself and wandered down the road towards your house and thus it was assumed the kid belonged to your house? I know when I was 3 and 4 i'd wander fairly far from the house (lived out in country). Dunno about when was younger (dont remember that far back )