I definately WOULD leave my 3yo (now 3.9 and would have done it from around 3.6) in the house alone when I popped the the shop 5 doors down (if we had one), both awake or asleep.
I have once left him (and 3mo) alone when I went to the chip van, 5 doors away as it happens, because ds2 needed to sleep and it was pouring down. My biggest concern was that ds2 wouldn't go to sleep and would cry (which he did) and that ds1 would try to help (which he didn't).
He was under strict instructions not to move off the sofa - when I got back (and was around 15 mins in the end) he was still on the sofa, and then told me that "I did get off the sofa to play with my cars mummy".
I was concerned but not enough not to do it, and was definately unconcerned about the whole SS thing as didn't think that mentioning it to a neighbour within hearing of other people in the queue was a problem.
Do you watch your 3yo all the time?
Mine plays in the garden or house where I can't see him all the time, ok I pop in and make sure he is ok (particularly if he goes quiet.....) now and again, but I wouldn't dream of having to be in the same room as him all the time.
I leave him playing downstairs, with access to the garden, while I shower - and if anything happened while I was actually in the shower, as opposed to drying off, there is no way I would hear it.
I'm not overly worried about people breaking in as we have a stinking great dog who would bite their heads off (actually she is a darling cocker spaniel who would want to be patted - but she doesn't sound like that...).
I also let him cut sandwiches (well my mum does, I just do it myself to get them made) supervised.
And if he had a friend a few doors down he is now at the stage that I would let him go round and play with them alone (though would ring and check ok first and that he is coming now, and would ask that the mum rang when he was coming back - and would watch him the first few times).
What I don't understand is how protective people seem to be, with older children more than 3yo's, but then go on about their 'lack of freedom'. To me the only thing that really is more risky now than when I was a child is the traffic risk.