IIRC the thing about "assault and false imprisonment" is that you can refuse to accompany the security guard anywhere; if you refuse and he/she then physically compels you or otherwise restrains you and it later turns out that the theft they are concerned about did not take place (i.e. no one had stolen any shampoo) then you have a case for false imprisonment. If you accompany them voluntarily then that doesn't apply. Although that's based on very rusty remembrance of what I was told twenty years ago, so I wouldn't suggest anyone's relying on it.
She does sound like an arse on a number of counts.
I have occasionally stuck my phone on vibrate into my bra if I have no pockets and am out in the evening with the DCs at home with a sitter, though. But only in a dark room, and never in the supermarket.