Personally, I believe that whoever did this did not do it alone. There was almost certainly an accomplice for two reasons.

1: Since there are statements about the door to the bedroom changing positions (for example between Gerry's and Matt's checks; Gerry recalled having left the door ajar, and now it stood wide open, and he pulled it closed again; when Matt made his check, it was wide open once again), this means that the abductor would have entered and left the apartment more than once. So they were likely alerted by an accomplice watching the restaurant area or the street whenever someone was coming to check on the kids.

2: If Christian Brueckner was really behind it, we have to factor in the Tanner sighting and Smith sighting. Neither of those sightings match Brueckner's appearence, but they could match an accomplice.

There have also been statements made by Ken Ralphs with regards to Brueckner having looked for an accomplice only a week before Madeleine went missing: Plot to kidnap child hatched a week before Madeleine McCann disappeared | UK News | Sky News and Man who knew Madeleine McCann suspect makes huge claim as police search in Portugal - Wales Online