TOPIC: Any idea what this is and what caused it?

Anything seem to make it a pattern, such as someone spraying some bug killer, or mildecide. or etc...?

Mike - There are after market films that can be installed on windows that some municipalities will accept to meet a "tempered glass" requirement. Look at the locations of the windows this is installed on, are they close to the floor, near a stairway etc. It may have been installed on the bathroom mirror for the same reason, just in case someone slipped and stuck out their hand to break their fall.

Due to the efflorescence I am thinking it had to do with moisture.  They buyer was there and was able to wash the residue off.  It has me puzzled.  I didn’t find any other evidence of elevated moisture, odor (other than what was in the pile of garbage bags) a small amount of surface mold in the bathroom around the ceiling fan but that was it. 

These windows and mirror are located in a family room with a wet bar no need for safety glass.

This one will remain a mystery. 

Thanks all for your ideas.