Terry, I'm not sure if manuals were provided for your Toilet...but not knowing which unit you have...here's an
Owners manual and
Application information to other units that might help.
If you have droplets on the fitting that would be a place to start. Are you able to see yours with a mirror also and able to get to it to tighten without taking it out? According to their troubleshooting guide, if it's leaking the connection was first and the valve second.
You also mentioned: "It seems to appear after use of the shower. I have checked all the drain fittings on toilet and shower. The pattern of the wetness is centered around the hole for the toilet supply." Most that have moisture in the basement as the result of the shower has found the leak coming from the shower faucet and not the drain. Moisture gets between the faucet base and wall and runs down the inside wall. With the moisture pattern 'centered' around the toilet supply...might just check the showers inner wall area in the basement after a shower to rule it out if not sure.
JLS, I hope that picture was taken before you removed and tightened it back up?!?!
workhorse