Hanging out at a friend's house is completely different from a date. They are 2 different things and it's good if you can spot the difference between the 2 just by your daughter's behavior and not just relying on her telling you (because, they don't always tell you).
Here's a perfect example from my experience with my son. My brother and I live in Florida. My brother's wife has a brother who lives in Colorado. They are very close to us and we usually end up visiting each other or our other siblings scattered across the USA. When we're all together we usually all camp out in either of our houses and the kids just sleep all together on a giant mat on the floor. They've been doing this since they were babies. Fast forward 14 years after my son was born and I notice my son developing a different relationship with my brother's wife's brother's daughter... so I sat my son down and we had a talk about this new development. They still all hang out all through spring/summer/winter breaks in one house... but we had to put new rules down regarding the sleeping arrangement. My son was only 14, his girlfriend was 15, close to 16. We still live in Florida, she still lives in Colorado so their relationship is mostly over the phone. I've known her since she was born so I know she has good character. I know my son too and how serious he is about his Priesthood and his desire for an eternal marriage. I was fine with their relationship but the rules we established included not going anywhere together without my other son to accompany. His girlfriend was Catholic. When my son got ordained a Priest, he flew to Colorado to baptize her. Now, my son is 18, she's almost 20, and my son is headed to the MTC in January while she is in her 2nd year at BYU-I. They haven't been anywhere without either my other son or her sister (who also got baptized with her) accompanying them. They just never felt a need to go anywhere alone.
It's good to have that initial worry - it prompts you to think deeply about everything before setting arbitrary rules or disciplinary actions.