Security theater

Yes, they can. They caught me with a menacing half-full bottle of water (oops). It was very clear on the machine. I had to go outside of security to dispose of it, couldn't just drink it on the spot.

Ok. I saw that post earlier but I didn't know if it was a plastic bottle or a metal one.

(if you're responding to me...) plastic

Mine were steel.  They didn't get flagged on our trip out, but they were empty.  I don't know if the TSA x-rays can see through the steel bottles to see if they are empty or not, or if they just dropped the ball.

For our return trip they cut off the lock off our our checked luggage. However, they didn't put one of those "we searched your luggage" notices in the suitcase, just placed the destroyed lock inside, and when the suitcase came off the luggage carousel it was half open. I will admit that I kinda figured they were gonna search it (we put the steel bottles in the checked luggage for the return trip and also had sauerkraut in there) but where is my notice?  And at least treat my stuff with respect and properly close the suitcase!