I got seen once by somebody I know on the way to a punt in a part of town I don't actually know anybody, but I simply just said I was on the way to meet a friend, and that was pretty much the end of the conversation.
I tend to have a good idea where my OH is most of the time, so I don't punt anywhere near where she might be. If I saw one of her friends or relatives that excuse should work anyhow, as non of them know all the ins and outs of my friends and social life. I tend to avoid getting to know her friends anyhow, I'm always polite to them of course, but I don't really want to be too friendly with them for precisely these kinds of reasons.
I just view it like, her friends are her friends and my friends are my friends, and this is a policy that has worked well for me.