It's the least conspiratorial theory by far, but because of the resort's seaside location, there remains a possibility that Madeleine's death was simply a tragic accident.
Reddit user glittercheese suggests this theory. "I think she woke up, left the apartment and wandered off. She either died either by drowning in the ocean (kids her age love water) or by some other form of misadventure. I do not think foul play was involved," they wrote in a thread about McCann's disappearance. They added as a later comment, "Drowning is the #1 cause of accidental death for kids age 1-4. They lack the capacity to understand the danger of water. It's why pools are required to be fenced in, at least where I live. The draw is very strong, and kids don't realise that they are in trouble until it's too late."
All reports on the case note that the family's apartment door was slightly open already when Madeleine was discovered missing, and the physical arrangement of the resort's apartments meant that there was relatively easy access to the outside road (where she also could have been killed accidentally) and nearby water. This theory suggests that there was no crime committed at all — just mistakes and circumstances that led to tragedy.