I had the same with a lugger bin that had been there for about 7 months, they used it for general rubbish.Another thread gone hopelessly off course!
50 years ago there was an open topped metal grain bin situated exactly where I'm sitting now. It was full, and rats were jumping in, feeding and jumping out at will, right up to the day that a few tons were removed from the bottom and sold. With the level in the bin reduced, the rats found that they could still get in, but unfortunately for them, couldn't get out again, which presented a bit of a dilemma as how to deal with them.
Someone had the idea of letting our dog jump in as well, which he did, and had the time of his life for an hour or so, as the rats would try to escape by burying themselves. Eventually we lifted an utterly exhausted dog out, covered in blood (hopefully not too much of his own), and collected 39 carcases.
Rats got in and bred, I wouldn't like to guess how many were in there.