[TeamG]

Rotten egg smell is often Hydrogen Sulphide, which is a chemical byproduct of decomposition. This could come from stagnant water, rotting rodent, leaves or debris.

H2S is detectable by smell at very low concentrations, but at higher concentrations is extremely harmful and potentially deadly. It also affects the sense of smell, so your observation of briefly smelling it is slightly concerning. Are you just getting a whiff each day, or is the concentration numbing your sense of smell and harming you?

Worth getting it checked. Have a good look anywhere that water, rodents or leaves can collect. My mother once had mice nesting in the heater box and only detected when they chewed through alarm wires causing a small fire.