One common place is the pressure/temp relief valve. But if that is
leaking, it should be obvious, as it should be coming out the end of
the pipe connected to it. If it's coming out the bottom, with no
visible leak at the top connections, it's time for a new one. Gas ones
typically have a life of 10-14 years or so. Depending on where it's
located and what happens if it goes suddenly, pre-emptive replacement,
even with no signs of trouble, may be a wise move at 10 years.