The Night Shift Stinks

So here's my dilemma, I work at a big-box electronics store during the day and graveyard shifts in the same place at night. I'm there from 10 PM to 6 AM, which is mostly just empty shelves and customers who need help finding things. Usually it's not too bad because I like solitude, but last week something happened that still keeps me up at night. I had a customer come in around 2 AM one morning, it was just an older couple looking for a gift for their grandchild's birthday party next weekend. Nice enough people, but they were kind of grumpy and asked about every single thing I could possibly recommend, which took forever to go through. The woman said she needed something interactive yet not too loud because the house has elderly residents. After helping them find a few things, she started talking to me about how we're all alone in there at night. She asked if it ever gets boring or scary and I shrugged, I mean, yeah, but what can you do? It's just my job. Well, later that morning when they were leaving with their basket full of toys for the party, she leans over to say goodnight. And then she kisses me on the cheek and says something like 'goodnight sweetie' before walking out with her husband. I'm left standing there in shock, what just happened? Was it a mistake or was that intentional flirtation? I didn't know how to react because it wasn't exactly what we agreed upon when she came in as a customer. And now this is eating at me all the time, is that a kiss on the cheek even crossing any lines of professionalism? It's not like anything happened, but it felt so out-of-the-blue and weird. Now I'm second-guessing my whole job because something strange like that can happen anytime to anyone who works alone late at night. How do you handle this sort of thing when a customer gets too friendly for no reason?