S10 indicates hot water demand. ie tap turned on. If you did have a leak on your hot supply to taps, it would most likely happen constantly, and probably prevent the heating from working, and it would have to be a considerable leak. If its more intermittent then it would point towards a boiler fault. This is where it gets tricky im afraid. I would suspect the aquasensor being faulty, but theres no guarantee without carrying out diagnostics. You could try unplugging the aquasensor while the fault is occuring and see if it stops