Could be receptacles that are 1/2 switched. Make sure you check both sockets when doing the test. It is common practice to put switched receptacles in upside down.
If someone swiched some of the receptacles it is possible they didn't remove the tab between the brass screws and now they are not switched anymore.

Many new homes are Pre-wired for future uses.......

Ceiling fan locations are wired for fan and lights....even tho only the fan is installed.....or even a blank cover at the ceiling location......

How about switched exterior outlets for the santa lites?

Half-outlet switches and unused ceiling light wires were the first things to come to mind. The other thing is that they might be three-way switches that were since disconnected, but you probably would not have current going through them if that were the case. Other than that, the junction box they control might have been covered by wallpaper or they might have wired a box for future application.

The only way to figure it out for sure is to simply test every single outlet, interior and exterior, (not just one plug in an outlet pair, but every single plug in your home), and create a map of what switch is connected to what box.