Water off/ still leaking

Two possibilities come to mind:

1. the isolation tap is allowing some water through - if so then it needs replacing as well

2. the leaking tap is allowing air in and so the water still in the tap side of the system after the isolation tap is draing out as the air enters to replace it.

Replace one or both taps.

Hot and cold lines can communicate through single lever delivery faucets. But the shutoff valves will stop this. Turn off the hot supply at the shutoff at each single handle control. Usually it is the kitchen faucet doing this, but it can be any where.