Look for any cracks, moisture or decay in the foundation, especially if it's brick or stone.

We had a radiator heat pipe fail and it took the tenants months to notice the moisture accumulation.  

If I may add, Never by no means ever waterproof or try and correct the issue with interior drainage systems, think about it your still allowing moisture in the basement, if anything now your increasing moisture from entering into the dwelling because now you just exposed below the foundation floor, its a cheap way of putting a i guess you can say band aid on a more serious issue.

Particle board if expose to moisture will bulge in few years.

The moisture - bleach - will seep into the grout slowly, all the better to perform the bleaching.  

Your FIL probably hadn't jumped into overalls and crawled underneath the house - any contractor that does and who has your interests in mind would be able to point out first signs of termites, mold, moisture and things like obvious cracks in foundation that old houses are famous for.

I have also applied it to the shower stall ceilings to avoid moisture issues & paint peeling.

I am NOT aware of anybody who has died from exposure to this mold.WHY MOLDS KILLMolds grow in response to moisture.  

If you’re finishing a basement, you’ll have to pay major attention to any issues with moisture.

The house was in way worse shape (including lots of moisture damage from an indoor marijuana grow), however our rehab was at a much lower finish level.