Taste it, if it tastes like poop then you know it's not piss! LOL I think it is from insects, I have it on my vehicles all the time and just QD it off. I think it comes from honeybees mostly.
I know exactly what you're talking about - I get this on my cars where I am as well.
The good news is that it always seems to come off easily and I've never noticed any marring/etching issues after removing this "mustard" substance (using Autopian removal standards of a few hours or at most a few days - it might be a different story of course if the substance was left on for a few months).