Common Mode Noise filtering for Diff Mode Circuit
How is the design to be placed relative to the ground plane used during testing? This is very important.

If you don’t know, you will need to find out because, how it is placed will influence what nodes on your “apparently purely differential and floating circuit” might have unwanted capacitive interactions with ground that produce (or create) common mode noise.

Whether those common mode noise levels are sufficient to contravene the regulations you wish your circuit to be compliant with is another matter. For this you need to investigate more and then (and only then) should you decide what measures you might need to apply.

But, there are some things you can do beforehand. For instance, if your perfectly differential circuit was encased in a metal case and that metal case were grounded to the 0 volts of your wires (in and out presumably) then you can begin to pre-think about measures that can be taken. 

This is because there is no physical orientation of the encased circuit that now makes the production of common mode noise any worse when considered against a ground plane. But there is a cost!