Is this a serious question? Look at the transformer. Is it marked as 16 volts?

The alert pen is first and foremost a voltage detection and warning safety device.
When you want exact voltage readings in house wiring and appliances, it's time to use the analog multimeter first for most AC work. When getting into deeper troubleshooting a cheap DVM may also be a big help but be aware an analog multimeter reading is usually the more "honest" one when it comes to home voltage readings.

This post was edited by laat2 on Tue, Jul 23, 13 at 12:05

Maybe I should have rephrased it. The volt pen says the low end is 50VAC but indicated voltage on the doorbell wiring that measured 16-18 VAC.

Is the pen picking up stray voltage to get to that 50VAC? or is it more sensitive then the label states?

A device like that doesn't see 51V or 50V and suddenly not detect anything at 49V or below. It can probably DETECT voltage below 50, but it is not very accurate at those levels, so they don't claim it will work in that range.

If you rub it on your shirt it will light up from the static electricity. Yes, it is possible it is more sensitive than the label says. As stated before, get a real meter if you are concerned with the actual voltage reading as opposed to if there is or is not voltage present.