The worst thing is the party. I have no idea how many strangers were in my house. For the next few months, whenever we can't find something of value we will have to wonder...? It's a major violation of trust. Picture coming home to vomit on your walls and a half filled drink sitting on the table in your daughters room. It would bother you. We keep noticing more areas of the house that are dirty or "sticky" from spilled drinks too. So you're right, it sucks that it's not in "tip-top shape", but the violation of trust is worse imo.
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.