I had similar symptoms and also found a slowly-seeping master cylinder. However, I also know that the clutch pedal was adjusted out too far at that time, which can screw with the self-adjustment of the slave cylinder. When I put in the new master it was properly adjusted, and I haven't had the issue since.

If you find no fluid behind the dust boots of either the master or slave, I would make sure that your pedal is adjusted as per Jack's video.