Though his posture on the van looked lazy, Leo was fully alert about his surroundings.
The reason he didn't position his body in a way that showed he was on guard was that he was also relaxing as well.
Staying guard didn't mean Leo could not do it in a way that pleased him.
Since Leo could see a large part of the surroundings they were in, should anything harmful to the people he had to protect appear, he'd have already been ready since he would have spotted them from afar.
Anything that he could not react to was something that Leo would not be able to handle in the first place, so he took things a little easy while maintaining his best form at that moment.
When David came out of his family car, Leo had already noticed him.
Leo did not move away from his spot and only took a short glance at David before removing his gaze from him to observe other surroundings.
Leo was happy to see David, though he did not express it.