Here is a pic that shows the spiral staircase at the basement level. Looking at the floorplans, this also seems to be the area where the 'back door' from basement to exterior is located, as well as the door leading into the house from the garage (PDF of basement level is also attached here).
Personally, I cannot imagine that they would have workmen coming into the house via the main floor with their workboots and such, to walk across the marble floors to the staircase, if there was an entry door toward the rear of the home leading to the basement?
The article says the HVAC guy followed the trainer and housekeeper into the house, but perhaps if he had been back and forth to his truck (to get tools/supplies, etc), he may have ended up exiting out of the basement door.. and wondering if perhaps that is how that rear door could have remained unlocked? He would have had no way to lock it after himself, unless it locked automatically or unless it was one of those cheap push-button type of door-handle locks (but even then, he could have not bothered, or simply forgotten to lock it).
It was a short time after his visit that the bodies were discovered, and reportedly by the time police arrived, that basement exterior door was found unlocked. Knowing there had been an HVAC worker right around that area just a couple of hours earlier, gives another potential explanation. However, I am sure the HVAC worker gave a statement to police and would've indicated if he'd used the door and/or if it was locked in the first place, or if he left it unlocked, etc., and hopefully police would've looked for prints. It's also possible the door's locked status was rarely checked on, being in the basement and such a large house. I guess the cleaning lady would know about those types of things. imo.
Look at picture below in conjunction with the PDF also attached to see where it is located in relation to exterior basement door, garage entry to the house, etc.
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