1. We expect a conductor to be a constant voltage throughout. The conductive surface (silver marks) is mostly a constant voltage, but sometimes you may observe a millivolts-ish difference between different points on it. What does this mean about said conductive surface?

Answer is explained in the explanation section below.

Explanation:

If the conductor has developed the potential difference. It simply means it is charged.

A conductor can be charged in different scenarios, let's discuss some of it and try to deduce the appropriate meaning for this question.

A conductor can be charged due to ions or

A conductor can be charged if it come across any current carrying conductor, so as to gain induced emf.

So, In both of these case, both can be very reasons for its potential difference.

thermal energy is the answer