I also worked polls in Philadelphia and we were also visited by non-uniform cops at the end of the day to do the (critical!) step of picking up the paper ballots. We got their badge numbers and names of course, but it was a little sketchy. The one who came by midday was uniformed, though.
Very interesting. Is it possible they were constables? They are very hit or miss, on uniforms and other aspects. Not sure if Philly has constables or not.
Part of the constable's duties is to open and close the polls. I'm not sure if they're required to be there the whole time or not. Our's is usually there. Sometimes he is in uniform, and sometimes not.
Here in Boston our officers were in uniform all day but they told us they drop off all the ballots out of uniform in their own personal vehicles, he surmised to be less obvious a target. I thought it was an interesting approach, perhaps not super effective but interesting.
