i know it is simple common sense to not explore alone. that is why i always explore with other people.
I explore alone...sort of anyway. Sure I have "lookouts" and use radio communications but I enter the buildings alone, not had anything MAJOR happen, one minor drug user situation a week or two ago but that was dealt with quickly and I have actually now been back to the site and spent some time talking to him.
Going in groups or even pairs is not always a good idea, 2 sets of walking noise, 2 sets of camera shutters / flashes / tripod sounds and when you are in groups, you will more than likely talk.
Ok from a safety point, going with somebody / people will always be a better idea but do enough planning and it shouldn't matter.
Krela is right though, mugging is a pre-meditated attacking method. It may seem like your attack wasn't planned but the same group probably done it before, were out planning to do it or even woke up and planned it. The second they said "Shall we" that was pre-meditated.
I was once walking around Bute Park in Cardiff, next to the castle in fact. CCTV everywhere. Group of 5 youths came up to me and asked me for the time, instant reaction, got the phone out and told them to it. Next you know, I am put to the floor and my phone is gone. Now all of the youths looked at each other when I got my phone out, all of them took the decision to take my phone.
Squatters can be ok at times. Sure, go storming the place and yes you deserve a good kick in - if you know they were in there anyway.
I have been to a few places where squatters have been, I knew they were there so knocked and asked their permission. Most of them are actually decent people that have just taken a bad turn in life at some point.
I have contracts not to show the photographs online / press but I have photographed buildings where the Occupy Protesters have claimed as squats, these people believe it or not are actually on the Anti Terror watch list with the Met Police and from what I saw, many of the homeless were taking part in these protests. All of them were harmless and actually polite. (that I met anyway).
In most cases people will react to what you act like. For example, the other day I was at my playground (The Billy Banks Estate) fire engines everywhere and in the middle of it all, metal thieves. Im taking photographs in a building, they come in. This could have gone very bad but what did I do? "Alright mate? Just taking some photographs, just ignore im here and stay safe." Their reply, "Ah nice one and you to - these are well dangerous."
Now that could have gone very different if I started to reach for a maglite, radio, phone etc. Although I do agree that in some situations there are sometimes no other options other than to run or get offensive.