Talk to neighbors and re-visit the apartment office.
See if you can talk to the apartment maintenance person and ask them to keep an eye out. Tell them that you will tip them if they locate your little girl, if they see her in another apartment and let you know, but that you will not tell anyone that they are the one who found your dog. Be certain to have someone else with you if you need to try to get her back.
Add a line to your "Missing, Reward Offered" notice that no questions will be asked for your dog's safe return.
I live in a condo complex and over half the time that dogs disappear, someone has taken them, usually for a friend or someone else that they "know". A couple of neighbors got their dogs back when the "friend" called them because of the phone number on the collar. Another neighbor was called and told that the person who was given her dog, wasn't going to give her back, but wanted the neighbor to know that her dog was okay, just with someone who "wouldn't EVER let her out of her sight!" The woman's boyfriend was at a party in the area, saw the dog on a chain and just unhooked her and left.
Per the caller, she knew that the little dog was loved because she was so sweet, so she called so the owner wouldn't worry......
Chihuahua's are very popular pets -the Disney movies aren't helping.
I hope that you are able to find her, but really don't think that she "froze to death", but that someone took her. People stink sometimes......