I found a hot wire in a junction box that is connected to a 120V light switch. When the switch is on the voltage on the wire is 120V, when the switch is off the voltage on the wire is 28VAC (bulb has been removed). This voltage will not light a bulb, but is present when the bulb is removed. When the switch is on, the voltage measures 120VAC and the bulb lights good. Is this a problem? And if so, what can be causing it.

Somewhere you have a cross circuit or a back flow. What else is on the circuit? 28VAC is very close to a 24 VAC transformer.

Does your light switch have any kind of internal(night light) light that goes on when the switch is off? If so it can create an issue similar to yours. If such is the case, the voltage would go away if you disconnected the hot wire on the switch. If this works I can explain in more detail later. If this is the case, it is not dangerous and is a way they can power up a circuit in the switch for the light.