This happened to my right shoulder a couple days ago while in bed still waking up, and my left shoulder today a bit after training.

I doubt I will get a response but hoping someone else here has experienced this and that its nothing to worry about.

When my shoulder was in a still position, I could feel a tendon move seemingly across the humeral head, as though it were out of place and sliding back into place despite my shoulder not moving. I have no idea what muscle it was, but it felt inbetween the anterior and lateral delt but more so the anterior delt, so I would say it was that.

I think I have maybe been stretching my anterior delt rather than pec minor, maybe that could be the problem.