Motan noticed this by observing with his peripheral vision while chatting with the receptionist.
Thirdly, after Motan deliberately took out that small box, the 'waiter' could no longer spare a glance for anyone else, and subtly moved his position to a spot which, while not close, was readily visible to the receptionist.
This was also observed by Motan using his peripheral vision.
Next, he was naturally caught by the receptionist to guide Motan, and during this process, the latter noticed many details that were inconsistent with his identity.
These included but were not limited to the cleanliness of his nails, his breathing rate, the details of his movements, his tone of voice...
And there was that incongruously large belly and leg fat of his.
So, the answer was already clear.
This guy had ulterior motives, and most likely was sent by the Thief Guild~
