What the heck are they doing?

putting up new siding? I'm not sure because there is no way to see that wall from outside..

unless you have keys to the building behind us.

oooh, a mystery!

sort of. it was definitely a nail gun and it was definitely the guy who's been doing work around here using it. that's all I know for sure.

How are you sure it was the guy doing work around there if you couldn't see him? (and is his name Schrodinger?)

I confess that there is a part of this puzzle I can see: the ladder going up to the roof of the building next door..

I see him climb the ladder, I hear the generator go on, I hear BAM BAM BAM, I make inferences.

although it is true that once he disappears behind our back wall he could be anyone doing anything.

The generator (compressor?) could be powering a mecha suit and he is playing handball with one of the Transformers .