The reason?
That man was not wearing any disguise and Arya could tell that it was his real face. Number 3 would not be so reckless to the point of appearing in front of other groups' members so openly.
He might be his assistant.
Arya was actually also qualified to take assistants, as in member of the organization who will work close to her but not given any codename. These special assistants had no fixed numbers but their capabilities might be even higher than the official members.
These assistants could be harder to trace back than the codenamed ones.
Because they have specific role and might not even break the laws and only did things within the available laws.
That way, it would be impossible to catch them.
'But it's more fun this way.'
If it was so easy to find these people, the number groups would not have caused a lot of troubles all over the world. Arya knew that some of the members of the number group was staying in the Sky Country Base, but she didn't know where.
