던데요

Polite 'I saw that..., and?'

Reports politely something you witnessed or were told, and leaves the sentence slightly open for a response: there was a whole list of toppings last time, he keeps hitting on me too, no one has ever recognised me, I gather some customers have this as a side. The 더 part means it came to you first-hand or by report, and the 는데 part hands the floor back. Compare 더라고요, which reports the same kind of discovery and closes the sentence.

Register: polite