Any suggestions for sensors to consider?
Not difficult at all: an lc oscillator will see its frequency shift as a human hand / face is approaching. So your mcu solution would be like this:
1) enable your watchdog timer;
2) use an exposed coil as the main oscillator to the mcu.
3) in the loop, count up a variable (do not initialize this variable) until it is reset by the watchdog timer.
the count will be smaller if a hand / face is approaching the coil.
The actual implementation will be more complex than this but the gist is the same.
The same principle can be used as a metal detector / finder.