I've been working as a cashier (customer service team member) for Woolworths for the past year and a half. I am a student so have been working on a casual basis. Ask me anything, but please nothing too personal.
Edit: Thanks for all the questions y'all, they were all fun to answer. From now on, I'll probably check this once or twice a day, so keep on posting anything you may want to ask me, I'll respond when I can.
She's right - you don't have a legal right to search it (regardless of signs saying otherwise or trying to make it a condition of entry). At best your colleague could try and perform a citizens arrest on the basis they saw her put those batteries in the bag and try to detain her until police arrived.
Not that I am condoning theft, but I've had some really annoying experiences with people asking me to unzip multiple compartments of a laptop bag to check what was in there and I'd rather tell them to bugger off.