Leaks in an on demand hot water system are likely to occur at the connections to the pump, which is typically placed under the sink located farthest from the hot water heater. Check for leaks around the pump as you would around any standard plumbing connections. Look for puddles of water under the sink and inspect the pipe connections to ensure that they are dry.
If a leak is detected, be prepared to shut off the water supply to the bathroom (or to the house if necessary) to conduct the repair. Typically a leaky connection can be repaired by using a wrench or pliers to unscrew the valve or hose connection. Place a bucket under the pipe and have someone turn the water on briefly to flush rust and water from the pipe. Replace worn valves or hose connections. Replace the teflon tape on the threaded pipe end and reconnect the valve or hose connection.
If the leak persists, contact a professional plumber.