I'm not saying that part doesn't really suck, but a 17 yo did watch your house for several days... I was a good kid, but I am sure I would have had a soiree given that opportunity. But, I would have done everything in my power to make sure the house was left in pristine condition. I would have been very serious about that. Leaving filth shows that she didn't care enough about your property. It's a lack of respect.
We've done this several other times too with a different girl watching the house. Each time we've returned to clean linens on the bed, clean floors, happy dogs etc. 17 doesn't automatically mean "gonna throw a party". Some kids do have respect for property.
She had a boyfriend. I'm 99% sure there was a lot of "boyfriend" time. I'd agree with your percentage, but would say it goes down if you let them know specifically that "people are not allowed over". You can have 1 person. I didn't do this with her, I should have. I should have let her know that the neighbors only expect to see one or two people here and only 1 car, otherwise they know to call me. I've learned a lesson. You can be the cool guy that doesn't care if they party, so long as it's clean. I'm not so forgiving.