Wire at the end of the circuit activates my tester at about 12-14 inches away
Those testers detect electromagnetic fields.
It's downstream of a dimmer
There are three dimmer technologies.  Variacs are a variable transformer which steps down voltage, giving a sine wave, but they are huge and heavy.  Rheostats are a variable resistor that adds impedance, giving a sine wave, but they make stupefying amounts of heat.  Both of these, being sine waves (as in (a) below), produce a 60 Hz electro-magnetic field. (50 Hz in the rest of the world).  Which is what that tester is designed to detect.
However both those dimming methods are totally impractical for the home.

So a third method is used: semiconductor dimming, which uses electronic switching to alter the sine wave (as in (b) and (c) above).  However, nature likes sine waves, and nature will treat a mutilated sine wave as if it were a conglomerate of various sine waves summing up. The science of this is called &quot;Fourier analysis&quot;.
Which sine waves sum up to (c)?  Many of them, at odd multiples (3x, 5x, 7x, 9x etc.) of the base frequency.  So 180 Hz, 300 Hz, 420 Hz, 540 Hz, 660 Hz. and so on. Janky shaped waves can create lots of these and they can be strong. Depending on the setting of the dimmer, these other frequencies will have different strengths, and depending on the room, will propagate certain ways.
The tester's sensitivity to these harmonics will depend on its own design.
I bet all these factors are lining up in a way favorable to detection.