>I can't think what on earth she could have done
An awful lot of information can be garnered from a quick 'Recce' ... Regardless of where the homeowners phone is. (Even a fairly bare hallway may contain goodies such as location of car keys! Lots of people still tend to keep keys by front doors. The 'I won't forget them' ethos)
Thieves want an easy life and knowing exactly where to go and where to get it gives a nice little in and out quick job.
Very much worth contacting your local police force (neighbourhood team if they have one. Google your local forces' website for information on how to report. (All Forces vary) an e-mail is often best if it is not immediately life threatening or a crime in process. It may be nothing, but if it is something your information may be a vital link in a chain.
Again forces vary, but if you are offered the service of an officer visiting your home.. Take it! They will often see security breaches you've missed or never even realised were a weak spot in your property.
And in future, yes sadly, it would be wiser to keep the stranger outside and offer to make the call for them the excuse "I have an ill family member inside and they are still contagious' is a good one ;-)
It may well be this girl did just genuinely need to make a call and that's it (many reasons as to why she might seek no other option than to knock on a strangers door, escaping domestic violence... Who knows massive spectrum of 'might be's! So don't be hard on yourself for being a Good Samaritan we can't see monsters everywhere everyday! But next time just be a bit more security conscious about it. Especially if you have valuables on display (cars out front, dogs etc)
Hopefully once having had a chat with Police you'll feel better about the situation.
Good luck xx