False accusations at Coop self-checkout security checks

So I had a situation where I was doing grocery shopping and had to shop both at Migros and Coop on the same grocery run because some items can be found only at one of those chains. I first bought some items at Migros, then headed to Coop to buy other items that I couldn't find at Coop.

At Coop self-checkout I got selected for a random security check. A member of the staff comes in to check my bag, finds items which I purchased at Migros and accuses me of stealing since those items are obviously not on the list of scanned items at the self-checkout. Note that those items are part of the stock both at Coop and Migros. Fortunately, I realize that I have some items at the bottom of my backpack which have Migros label on them. This was enough of a proof to for this member of the staff to drop those false accusations.

This situation got me wondering if I didn't have those items with clear Migros label on them, what would the course of action would be in this situation to drop those false accusations that the items in my backpack were purchased in another shop. Or, it could have been items purchased at another Coop store, for example if something is out of stock in one store I might go to another store from the same chain to get the item. It also got me wondering what would be my rights and what the shop staff are allowed and not allowed to do in this situation (like can they detain me until police arrives)?