Floating input in Op-amp Readout Circuit?
As people have said in the comments, the floating input is U3C pin 10, and R10/R11 don't have any influence on that.

The article Avoid Common Problems When Designing Amplifier Circuits from Analog Devices covers this pretty well. The situation with U3C is like Figure 1 of that article. (N.B. I don't have any expertise in EEG electrodes but I presume it's safe to model them as capacitive sources.) As shown in Figure 2 of that same article, the best solution is to provide a DC path to ground, ideally a high-value resistor.

As also mentioned in that article (and as you hint in your question) it is good practice to have both inputs of an op-amp see the same impedance, to minimize offset errors and other problems. So this would imply that the circuit needs two other modifications:


The path between the U3C output and the inverting input should not be a dead short, it should be a resistor of the same value that was just added to the + terminal. In practice this would mean finding a resistance that is high enough to work on the + terminal so the electrode still functions properly, but low enough that it still makes a good feedback resistor. I'm guessing 100k would work, though that's mostly a guess on my part.
R12 should be set to a value equal to R10//R11. The exact value is 952 ohms, but even a value of 1k ohms would probably be pretty good.