While doing some testing on my RPi 3 today, I noticed that the little fan in the Pi's enclosure kept on running even after I removed power to the Pi. The fan is connected to GPIO pins 2 and 6 (normally 5VDC across them), but I was reading 3.7VDC with no power applied. So I unplugged the HDMI cable feeding the monitor and the fan stopped, the voltage went to zero. Reconnected the HDMI cable, 3.7 volts again. I unplugged the 12VDC power to the monitor and the volts went bye-bye again. The monitor is one of these: https://www.amazon.com/gp/product/B01I1 ... UTF8&psc=1
When I tried the same trick with a somewhat more reputable Samung TV/monitor, there was none of this nonsense: with no power applied to the Pi, there was no voltage on the +5V pins of the GPIO header.
I don't know how happy the Pi 3 is with 3.7V Vcc but I bet not very. Jack says, that little monitor is dodgy. Use caution.