The use of the "hollow tube" manifested an audible sound effect when Sulu was absorbed. When Tamar was blasted by the "hollow tube" there was clearly a blast sound effect with sparks and smoke coming out of the tube. Spock was correct in that he could find no obvious visible mechanism on initial examination. But later the tube was explained as part of a larger advanced tech wireless mechanism and even called out by Kirk as the advanced tech is consistent with what they've seen so far.
SPOCK: Fascinating. This is merely a hollow tube, Captain. No mechanism.
...
KIRK: A lighting panel.
SPOCK: Amazing in this culture.
REGER: Comes from a time before Landru.
KIRK: Before Landru? How long ago was that?
REGER: Nobody knows positively. Some say as long ago as six thousand years.
SPOCK: It took a very advanced technology to construct a device like this. Inconsistent with this environment.
KIRK: But not inconsistent with what we've seen. Security.
KIRK: Those staffs. Hollow tubes. Antennae for some sort of broadcast power. What is it, Mister Spock?
SPOCK: Strong power generations, Captain. Near here, but radiating in all directions.