I have seen almost no discussion of the furnace room on the lowest level as a place for the perp to hide and wait. How much attention do people, anywhere, really pay to the area where their furnace is located? Change the filter once a month or every three months, depending on the type of filter used, whether the homeowner, maintenance guy, or tenants are the ones changing it. Turn the heat off in the spring and on again when the weather starts getting chilly in the fall. Maybe run the vacuum cleaner in the furnace room a couple of times a year to get the cobwebs and dust.
The following scenario is MOO:
Perp gets into the supposedly empty house-Perp doesn't realize that B. and D. are there because they are asleep and their lights are out. Hiding upstairs is too risky, plus location of the dog is unknown to Perp. Perp is familiar with the floor plan either from Zillow or because Perp has been in the house previously. Perp sits or crouches in furnace room or behind the furnace and hears the other 4 arrive home. Waits an hour or two until things upstairs are quiet, then proceeds upstairs to the victims' rooms.
On a side note: the house my siblings and I grew up in had a large furnace in the darkest corner of the basement- large enough that 3 people could easily have hidden between the furnace and the exterior wall. As a child, it terrified us to go into that area of the basement to get the laundry or canned goods that my mom needed in the kitchen.