A creak.

Completely entranced by the scene, and her own needs, Beth didn't notice the creak in the floorboard behind her. When she heard the second creak, she froze, hand in the proverbial cookie jar. She got the distinct, terrifying feeling that someone was behind her, and she slowly turned... to find Tim standing at the end of the hall. His hands were in front of him, palms out, in a gesture of peace.
"Oh..." she couldn't really process what was happening, and the sounds of sex were unmistakable in the room behind her. She tried to pretend that she wasn't doing exactly what she was doing, hiding her hand behind her back. "I... b-bathroom..."
"Over there." he whispered, with a shit-eating grin forming on his face. There was no fooling him; he had clearly seen what was happening. Oh my God...I want to die. I am going to die of embarrassment. She practically fled into the head, sliding the door shut and sitting down to finally pee.
How was she going to face this? What did Tim think of her? Why had she not been able to control herself? She finished up and sanitized her hands. When she slid the door open, Tim was standing in the doorway. The sounds of sex still wafted from the nearby bedroom.
"Show's still going..." he whispered. "I have no qualms if you want to keep doing what you were doing."
"I...I...." she had no words. She wasn't gong to simply continue watching and masturbating.
"Don't be ashamed. We all have needs. It was kind of hot... watching you do that." She couldn't help but notice his bulge. Did I do that?
"I don't think...it would be....app-appropriate..." she squeaked.
"Oh definitely not...but still." the sounds got louder. "It sounds like it's getting good."
Beth gave an involuntary whimper.