Shendon Pod is a plant used for several purposes. There was a time when people extracted its juices to prepare perfume since the fragrance of the plant was quite attractive and made people stand out.
However, slowly, the popularity of the perfume went down once the use of Shendon Pod's pulp came into knowledge.
A lethal poison.
Soldiers began extracting the pulp of the flower and used the extraction to coat their weapons with it, which can cause slow death even by a shallow cut to their enemy.
That's why when Celestria said that she smelled Shendon Pod, Alex suspected that there was someone who was there to cause harm. He first suspected that person to be after the Princess, however, if they wanted to, they could have tried to harm her when they were in the crowd.
After that, Alex kept a close eye on his surroundings, and only after Sarah arrived at the eatery did he realize the main target of those assailants.