Sparks on grounded metal bodies on lighting strikes in the nearby (two miles)
Short answer, lightning carries so much energy in the form of electromagnetic waves, it can create current in objects a few km's away.

Lightning can create electric fields that are more than 100kV/m directly, shown from data taken from a plane-strike.


Source: http://www.atmo.arizona.edu/students/courselinks/spring15/atmo589/lecture_notes/apr03_2015.html

These taper off with near field and far field rules. If the lightning were obeying far field rules the electric field would taper off with the distance squared. These fields can still can be quite high even only 1km ( or a few kms) away from the strike, a 1m conductor would experience capacitive coupling of a few hundred volts a few kilometers from the strike.

I expect that the stove pipe is a little longer than this.

Here is the fields from a simulated strike. Even the magnetic fields are in the A/m range. Every conductor every wire could also have current inductively coupled into it.


Source: Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA‐based lightning location systemsenter link description here

At even a trace of 1cm could experience a few volts across it, which may be why your LED's are lighting up. 

Apparently nearby mountains amplify electric fields as shown in this simulation (although the wave and the mountain in the simulation are not on the same scale, perhaps this is leading to even more amplification of the EM-waves):


Source: Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA‐based lightning location systemsenter link description here