According to all the trim on my vehicle, I have an XE/V6 Xterra. So you may be correct that I do NOT have an alarm.

If there was an alarm installed, where would I find it?

I'm curious about the toggle switch myself. Anytime I've seen a switch down along the driver door, it was always for a Valet switch. I'll have to take the kick panel off and see where it goes too.

At the moment, I have no idea what it's used for. I don't think the previous owner does either. If anything, maybe the Original owner had an aftermarket alarm installed and then yanked it out before he sold it. So that's why there is a toggle switch left behind.