I have an older home, built in the 40s. Whenever I replace an outlet I check to make sure with a pen voltage meter that I am wiring the correct hot and neutral wires, because most are not colored coded. I went to replace one recently, with two lines coming into the receptacle. I went to test for the hot wire, but all 4 wires are setting off the voltage detector. Is this strange? Seems like it.
When testing circuits like this, you really need a meter, not a wonder stick. The voltage testers indicate the presence of an electric field. In a box like the one you have there is an electric field and it can spread to the un energized wires. The tester only indicate the presence of voltage, not how much. Meters will tell you the actual voltage on the lines.