I've noticed that a couple of my Japanese friends, when talking about their plans, might say something like 「友達に会う」, "I'm meeting a friend", even though the unnamed "friend" is someone we both know. It's a little jarring when I know from another conversation exactly who they're meeting, or if I find out later who they were meeting, and it's a mutual friend.
What I expect from my own experience as an American is, for example, for them to say something like 「ゆかちゃんに会う」, "I'm meeting Yuka", because to me "friend" or 「友達」 sounds like someone I'm not expected to know.
The effect is that it ends up feeling like the people they're meeting is being deliberately kept secret for some reason I can't fathom. But, is it simply normal to leave out the names even of mutual friends as unnecessary information?