Honestly, with her being about 5-ish months based on your observations I think it may be possible someone "let" her out. She may have been a Christmas Puppy who wound up being more work than the owner expected. Obviously, keep trying to find her owners but if you know you aren't ready to take on another pup, I'd start getting a hold of rescues and let them know of the situation. I'm positive that they'll be able to help you
thanks for stepping up for this little lady.