Bear was only vaguely surprised by the dog that ran into them seeming out of nowhere, but there was no aggression, only simple surprise. She was not the source of the food, so he had no intention of pursuit.
Mourn, on the hand, had taken off after the strange dog into the building. Her curiosity had gotten the better of her, and the excitable mastiff hadn't been able to contain herself.
"No, Mourn! Get back here!" Bear snarled, lunging to try and catch his sister. She didn't know if this dog was aggressive or not! If his sister was hurt, things would get ugly. He entered the building behind his fluffy furred sister, but she was unseeable in the dark. His eyes blinked to try and pick out her shape, and finally the white mark on her chest made itself seen.
"Hello? We won't hurt you, I promise!" she was saying in a soft tone tinged with excitement. Bear stalked up to her and growled softly.
"What are you thinking?"
"Hush Bear! You'll scare her away!" Mourn snapped before searching for the unfamiliar dog yet again