I have left Jess at home at 16 and its been fine but my dad lives nearby! I have had her stay at a friends too and have contacted the friend's mum and exchanged mobile numbers and said if they try to change any plans call me!!
I think you should trust her to still have her key, I know wild parties seem to be on the news all the time but truly I very rarely hear about them from DD and none which are while the parents are away and she tells me things. It doesn't seem as much of a big deal to have parties while peoples parents are away now as it was when I was a teen, DD goes to heaps of parties with alcohol and worse where the parents are home.