Lamp between switches in multiway-switching
I'm guessing your are using a proximity type tester. Some are very sensitive to any electrical current even static charge. So if you have a wire next to a hot wire yet is disconnected from the circuit, mutual induction will cause some trace voltage with very small (milliamp) current and yes it will register on your sensor. Thes type of sensors will tell you current is present but they don't do much more than that.

If you are concerned, try using an ammeter it has settings that will give you voltage and current strength.