3 guesses

as to what is located in an inaccesible location under the panel. :mrgreen: (Ding Dong)

Surge suppressor. . .

Excellent guess, as it is I can only hazard a guess as it is inaccessible!

I guess doorbell transformer.

If it is connected to both legs it is probably a surge arrestor, but looking at that white wire, if it is connected to 120 it is probably a doorbell txformer.