When there are two or more of those things in close proximity, I think a different pre-recorded voice should be used for each. At the very least, it would offer some variation. It could also avoid confusing people who are checking out at nearby stations.
The theft prevention measures on most of these models are limited to (a) a cross-check between weight detectors positioned beneath the "unloading" and "bagging" areas, to make sure that each item you unload is scanned before being placed in the bag, and (b) an employee, who is usually fixing the tape at another station or helping someone who can't figure out how to operate a scanner (or who is confused about the voice prompts; see item 1).
It seems to me that a really easy way to shoplift items in your basket if the employee isn't paying attention is to remove a handful of items at the same time, scan one of them, and place them all in the bag.