I heartily +1
@buggi post above and will try to do my best not to repeat it.
A couple of points though...
I can think of a scenario where seven officers turning up could be entirely possible. Most city forces (you said it was a city) put out extra officers for the 'night time economy', that are mostly just there for the drink fueled violence. They work from vans with (traditionally) one sergeant and seven PCs. We have to ask these vans to assist us every weekend now with jobs that aren't really in their remit, but we've run out of units. This could be a plausible and likely scenario for me of seven officers turning up.
You don't know what the call to the police was. I'll give you a cast iron guarantee that there was a call. Quite a common one we get regarding noisy parties is "There's loads of shooting and it sounds like someone is being killed". When we get a report like that, even when it's likely to be an over exaggeration, we are going in to check, and yes, every room.
I don't want to go much further on that point though, as I am guessing and hypothesising the whole thing. While some may be happy to give certainties about what the police could or couldn't have done in your situation, I'm
not overly happy to without the whole picture.
Lastly, and let me make this really, really clear. If your daughter was at an address where 15 and 16 year olds were knowingly left alone, knowing there would be alcohol and suspecting there would be drugs, then a visit from the police is not surprising at all, and perhaps the view from my ivory towers is distant, but I'm shocked you allowed her to be there knowing those facts.