They actually covered this, the unit would alternate between existing and not-existing (presumably the alternation would be on the time wave pulses).

I work in a smaller datacenter, it is rather uncomfortable. It's mostly hot everywhere except for the cold rows where it's fairly pleasant (temperature-wise). You can't hear much of anything over the noise of fans and stuff. Fortunately we have a small cluster of offices right next to the DC which is where I spend most of my shift.

I can't speak for other chains, or even other stores, but at the Kroger's I work at there are very few cashiers actually trained to run the self-checkout. Most of the teenagers are too young (cashiers under 18 can't sell alchohol (/sigh @ Michigan)), or are only there for the summer or can't work til closing (when self-checkout is the only thing open) so the managers don't see it as worth the time to train them. Many of the other cashiers absolutely hate running it and/or avoid being trained on it.