On Sun, 27 Nov 2016 13:45:00 -0800 (PST), David wrote:

Another alternative, if the boiler is feeding an indirect hot
water cylinder, is a leak from primary to secondary within the
cylinder. This would be difficult to spot unless the leakage
rate is high enough to cause the header tank to overflow, which
would happen only if none of the taps in the house are run for an
extended period...

Header tank?