OK, I have isolated (turned off both hot and cold supply) the washing machine, mixer faucets in both master bathroom lavatories and the master shower, the kitchen mixer faucet, the third bedroom mixer shower and the laundry mixer faucet. There are no more places with mixers. All the rest are separate spigots for hot and cold water.
The problem shower/bath (which is not a mixer valve) still maxes at about 112 degrees, but the lavatory faucet next to it reaches 150 degrees.
Is there any safety valve or other such item that could be installed out of sight and which could cause this cross-over problem?