M&S on Grafton Street beeped me at the exit

I visited Marks and Spencer store on Grafton Street and I bought some pasta bags and sauce bins. But I was beeped at the exit. How come I often get this beeping in the stores? It was the second visit for me today and first was the lunch purchase and nothing happened at all. And the second time was just for the dinner stuff purchase and I went only to the downstairs only.