Some smells can stay in the walls, central air filters, or in poorly cleaned (or not replaced) carpet, as we all know too well. The burnt wire smell would worry me. Is your wiring up to date? and are there any overloaded outlets?
I've had the infections smells too.
I cannot stand the smell of pickles now.
And on the other hand, my old house did have smells. These were very odd as they were like a cold spot, you could find definite borders to it. One occasion I had been sitting on my bed drawing and decided to get up. When I leaned to the right my face went into this horrible dead smell. Leaned back, no smell. Right of it, left, above, below - no smell. It was a roughly a 2 foot diameter ball of noxious smell.
We'd also smell old fashioned lavender smelling perfume from time to time. No one in the house used that or any candles, and we didn't have close neighbors.