Arduino as voltage monitor shorts to ground
A voltmeter does not have a ground reference. An Arduino plugged into the USB port would have one. Conceivably a battery-powered Arduino might work, however a safer solution would be to use an opto-isolator to isolate the voltage being tested from the Arduino. Something like this:



In this case you connect the "data pin" to the thing you are sensing. If it is at 5V then the LED will conduct (taking around 7 mA) and the transistor in the isolator will drive the Arduino pin LOW. If the data pin goes to zero volts, the transistor will not conduct, and the 1k resistor will pull the Arduino pin HIGH. Thus you can detect if the pin has changed voltage.

The 560 ohm resistor is a current-limiting resistor for the LED in the isolator. A 1k resistor would probably also work, reducing the current draw on the data pin to around 3.8 mA.