My only explanation- the person was on a tight time budget. That person knew that Ana was talking to her friends every day. So the break in routine would make friends anxious. The abductor wants to buy these two days, when he is not looked for. Somehow, during these two days, he is vulnerable. Later, not so much. In hindsight, it will be hard to check. But, his alibi doesn’t stand if it is checked within these two days. JMO.
It happened in the reverse, but it worked. What worked for him was the mentioned Barcelona trip, and the Sat-Sun time of the abduction. The Spanish police says, let us wait till Monday, maybe she’ll show up in Barcelona. Then Ana doesn’t show up, and it means that she has disappeared.
What doesn’t work in the plan is the fact that Ana is very close with her friends and they know her far better than the person thinks. So they don’t fall for just any text and instead, they get very anxious. Sanna is important. The brother is “family”, and families may have problems. But a very calm, careful person who says, “this is not Ana” or “she is my best friend, this is not her” is the collateral witness everyone can only dream about.
We don’t know what else happened. It is obvious that the person is a planner, that he has a flimsy alibi for the two days around Ana’s disappearance, and that while he knows of her routines, he couldn’t predict that his texts would make her friends anxious. Very sure of himself but not understanding people well. (But let us think the worst, a SK on Bumble, would he even think of sending texts? I don’t think so.) I think this person has known connection to Ana.
If I were Spanish police, I’d check the alibis of everyone they spoke to. Maybe someone on Bumble is traveling now, check the travel time. I guess Spanish police can talk with Serbian too. Everyone around Ana needs to be checked for the validity of her alibi. (Her friends as well, just in case.)