The Is Ammo Low branch will search for ammo if the agent is close to running out:

Before the agent can search for ammo they first have to know what type of ammo to search for. The first conditional task is parented by a Selector so the agent can either search for non-power weapon or power weapon ammo. If the first Is Ammo Low condition returns Failure then the branch with the Random Probability will run. The agent shouldn’t always search for power weapon ammo so the Random Probability was added to introduce some randomness. If either Is Ammo Low tasks return Success then the Search For Ammo task will run and this task will return Success as soon as the agent has picked up more ammo.