From your description of switch wiring, it sounds like you have one of the switch leads attached to +5 V and the other switch lead to an input.
As noted in comments and answers to "Why all unset pins of my Arduino Uno are outputting 2 volts?", such an input will read high when the switch is closed, and will read randomly when it is open, because an input is a high-impedance device that can accumulate charge:
... input impedance typically can be taken as 100 MΩ for design purposes. ... Each pin has a few pF (5-10?) of capacitance ... which in general is small compared to stray capacitances of attached wiring.
When stray electromagnetic fields charge the pin and stray capacitances, a small voltage develops on the pin. You could connect a 1 to 10 MΩ resistor between the pin and ground, to avoid the [voltage] you are seeing.
The easy fix for the problem is to attach one switch-lead to ground, one to an input pin, and initialize the pin with INPUT_PULLUP pinmode instead of just INPUT.
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