No need for forced entry , the patio door was unlocked . The front door next to the open window can be opened from the inside . IMO he came in via the patio door , opened the window to check his exit was clear or possibly to hide the smell of ether . The crime scene was not properly sealed off and multiple people went through it and given what we know how well the PJ handled the HaB case not surprising.
CB was a prolific burglar as well on the Algarve , yet he was never caught . In terms of forensics CB was well aware and likely was wearing gloves and something on his head. We know he was forensically aware as he wore condoms in the attacks on HaB and DM . He was also prepared to stay in jail rather than risk losing his stash at the Yellow House , that backfired when he got turned over , he even wrote to his girlfriend asking her to clear the place of video and camera equipment.He then tried to bury a large haul of csa under the presumption no one would dig further but he was prepared to , such was its value to him .
People who are quick to judge HeB should remember it was a report of an alleged viewed video of an elderly woman being assaulted which led to the BKA asking the PJ for any cold cases. This then led to the DM case being unearthed and CB being convicted .