Why does my heat pump smell like burned scrambled eggs?

Odd situation. We came home last night, and our house smelled bad. The electric heat pump seems to be putting out an odor that smells like burned scrambled eggs. (Not rotten eggs like with propane, but actually burned eggs.) The heat's been on pretty much since October, so it's not just the normal dust burn-off. I have cleaned the filter and coils. It looks like there may be some mouse droppings (delightful!) under the coils, but I don't see a nest. Any ideas? Can't be CO as it's all electric. Oy!