As I said it might not be smell of gas after all, that is the reason I suggested turning gas off at the ECV, and then that would clear any issues with your boiler. Do not forget smells of dead creatures such as mice decaying can often resemble like a faint gas leak, and is always difficult to pin point, especially if it dies in cavity walls, or under suspended floors, or in roof space, this smell could linger into your utility room, have you also tried to keep its door open with the rest of the house to see if the smell disperses, and how long does this smell last after your boiler swivches off, and how soon after it switches off that you start smelling this, could be that heat is keeping.
A friend has gone abroad on holidays, giving me his house keys to go in and remove his post and to turn curtains and lights on during the dark ours, I went into his utility room, and it smelled like a gas leak, but it wasn't as he turns his ECV and water valve closed before he goes for a lengthy period, so it could not have been his gas, but possibly a dead mouse, the smell of which resembles that of gas, as it must have died a while ago and the smell isn't very strong so as to distinguish it as a dead mouse or completely as gas. I just opened his roof light slightly so as to vent the smell. And of course it could well be his drains as well.
If the boiler did have a faulty seal or gas leak, it would not be intermittent. and not smell anything for 4 days and again a dead mouse smell would be present all the time until it decays completely of any flesh, so could well be your drains, where the smell lingers in and accumulates in utility room, when heating shuts down, possibly drawn in by room getting cold etc, (air currents due to temperature shift) .
