Dead rats smell the same as any dead mammal. More likely a mouse, which I'm sure they've got lots of. I remember when the mice in Borough High Street Sainsburys used to be quite comfortable running about the shop when it was full of customers. I don't buy the unpacked veg from there.
This dead animal stink is getting worse.
I talked to a manager there a while ago. I began with mentioning in a conspiratorial tone that I work with rodents. It worked because after a quick shifty look round he admitted it was mice, tons of them. They are under the fridges. I said they really need to be cleaned away and he squirmed and said 'yeees but if would have to be done at night....' Obviously too difficult for them. Don't breath in I suppose.