wow what a stark reminder, certainly an odd experience all on its own, i guess the best thing to do in such a situation is to act as casual as you possible can, saying weekly shopping is a great thing to say, or something like, well i just care about the coins in my pocket, or this place does really nice <insert nice item here>....even something as simple as what's a prepper?...or crack a joke about how it sounds funny, anything to throw them off the scent or to make you seem ignorant!
at any rate that is certainly not the usual experience and is exceptionally suspicious...if you are on good terms with the shop owner, as it sounds like you are, i'd simply politely enquire as to if she's seen this woman before, or if its a usual sort of thing to be asked, and even whats a prepper, again to throw off any suspicion!
make a good mental note of what sort of person asks this sort of question, from small details like wedding ring or not, stance etc more often than not small details like odd bulges in clothing andthe way they hold themselves can tell you a surprising amount, and wether or not they are an official sort
also TOF definately had the right idea with the box of chocs or wine, definately a good idea!
anyway thanks for the stark reminder of the times we live in, shall strive to be more careful from now on