Mysterious musty odour in hallway but can't find any signs of moisture

A musty or earthy smell is often one of the earliest signs of a moisture issue, even if there are no visible signs of water damage yet. The fact that the odour becomes stronger during hot weather suggests that heat and humidity may be amplifying the smell coming from hidden materials or spaces within the home. Some common sources include moisture beneath flooring, damp subfloors, hidden plumbing leaks, water intrusion around doors or foundations, wet insulation, or even mould growth inside wall cavities. If the smell is concentrated near the front hallway, it could also be related to the entry door assembly, nearby exterior walls, or moisture migrating from a crawl space or basement below. The mould you've already noticed on an upstairs wall may indicate that the home has broader humidity or ventilation issues that should be investigated as part of the same assessment. A good place to start would be with a qualified building envelope contractor, restoration specialist, or mould and moisture inspection company. Tools such as moisture meters, thermal imaging cameras, and humidity testing can often identify hidden problems that aren't visible during a standard visual inspection. One thing we often tell homeowners is that persistent musty odours almost always have a source, even if it's hidden behind finishes or under flooring. Finding and correcting the moisture issue early can help prevent more extensive repairs and indoor air quality concerns down the road.

That smell rubbing off onto your bag is the real clue here — that's active mold, not just a musty smell, and six months plus getting worse in heat lines up with a moisture source that's still there, not gone. I wouldn't chase this yourself much longer. Sniffing carpet and floorboards won't catch what's going on under the subfloor or inside the wall near that entryway, which is exactly where I'd suspect it's coming from — front hallways are notorious for water sneaking in around the door threshold or from grading outside sloping toward the house. Skip the general contractor and skip insulation guys for now too. Call a mold/moisture inspector first, someone who can actually put a moisture meter or thermal camera on that hallway floor and wall, and check if it's connected to the mold you've already got upstairs. Once they find the actual source, then you bring in whoever fixes that specific thing. Doing remediation or repairs before you know the cause usually means paying for it twice.