Bellatrix wasn't hiding. Not yet, anyway. There had been a bigger fuss when she had attacked Helena, and until she saw some action from the naysayers, she figured she was safe enough. Still, she was scouting out possible places for her to go.
She had found a few places by the time Tonks caught up to her, and as soon as she heard her niece's voice, her hand twitched for her wand. She could grab it in a second if she was in any danger, and as she looked at Tonks, her face showed no emotion, except a slight annoyance. "That would be good advice if I had been looking for him. I wasn't. So you can go and run along now," she said with a shooing gesture. She was tired of these people. If she could discover a way off the island, she would take it immediately just to be rid of them. She was around entirely too many disowned relatives.