Carpet needs to dry out within a day or two of getting wet, or mold has probably already started into the padding. In older cities the obvious cause is a flood or a burst pipe. In Irvine, it's often quieter: a slab leak. Plenty of homes here sit on a concrete slab with copper supply lines running through it, and once one of those lines starts to weep, the first sign is usually a damp patch of carpet near a baseboard with no water visible anywhere else. By the time the smell shows up, the leak has usually had time to feed the padding underneath before anyone noticed.
Condos and townhomes add another layer. In villages like Woodbridge and Turtle Rock, a lot of the housing stock is attached or shares walls, and a leak on your neighbor's side of a party wall can wick moisture into your carpet before anyone next door knows there's a problem. HOA agreements typically cover the exterior and the roof, so the carpet inside your unit is on you, and that split leaves homeowners arguing over responsibility while the mold keeps spreading. There's a seasonal angle too: Irvine's marine layer brings damp, overcast mornings through most of the summer, and a closed guest room can hold that humidity long enough to grow carpet mold with no leak involved at all.