Although it was repeating an old trick, hiding that package in the toilet tank of a public restroom would also work.
But the adjutant had just gone to retrieve the item, and Xu Shuo had specifically instructed him to leave some "inadvertent" traces behind when he left.
Room 311, however, was unoccupied. In fact, after being searched once, such vacant rooms are slightly safer than public restrooms.
After all, even if no traces are left, what if the Capital Commandant, after having finished searching the hotel lobby, suddenly remembers that the hotel restrooms hadn't been searched?
It's possible the other party might search the empty rooms again.
But as the layers increase, the restrooms actually become quite risky, whereas the empty rooms are relatively safe.