My guest who checked out this morning messaged me to say the toilet was leaking from the top. This was helpful because I was already in the bathroom stressing about the water on the floor around the toilet and contemplating if I could manage to hold off calling the plumber until tomorrow (Monday here).
When I flushed the toilet water did indeed come from under the tank lid. I made the mistake of lifting the lid and finding water spraying out! So I quickly turned off the supply valve and found that this white tubing had just come out of place on the float mechanism. Whew! Easy fix. But now I know to periodically check and make sure it’s still snug.