Moisture Reading Between Boards

[KevinC76]

Get some dye and put it in the toilet and down the washer drain. You need good quality dye for this, not food coloring or those little tablets they sell for toilets.
I've had good results with this: https://www.homedepot.com/p/Eco-Clean-1-Gal-Green-Water-Tracing-Dye-GREENDYE-1/305766181

I'm guessing not to do both at the same time, right?

I should have added that there was no evidence of leaking from the tank into the bowl, which is unsurprising given that the level has dropped so much.

 

[KevinC76]

Sorry for the lack of updates--it probably seems ungrateful.

We were in the hospital for a new baby and haven't done much with the bathroom.

The toilet bowl water level continues to decrease, perhaps a bit more slowly. The moisture readings generally have decreased a bit, although that may just be because I have a fan blowing on them. (The readings are lower on the side with the fan than on the side without a fan.) Because we were gone, we haven't used the toilet, but with rainy weather approaching, we're going to see if that causes a spike in the readings, which would be a sign of a venting leak making its was down (one of our plumber's guesses).

Thanks again to everyone for their help. I apologize for being out of touch, but babies come on their schedule, it seems.