I am a firm believer in human intuition, which seems to have served me well over the years. Follow the messages your 'other' sense gives you; if an area or person gives you the creep, bad karma or bad vibe, just get away from it/him/her.
I was in La Paz, Bolivia, about to take a bus down into the Amazon. Now I have never actually had anything stolen while I travel as I'm pretty careful, but as our bags were stowed onto the bus my senses were screaming that we were about to get done. Nothing happened, we boarded, departed ... and then I found my camera had been nicked from the overhead luggage rack. This, despite the fact that it was strapped and locked down tight and nobody got on or off the bus. (It just goes to show that if someone wants something enough, they'll get it.) Over the years I have pondered just how they actually got it, and have come to the conclusion that they must have had hidden trapdoors in the roof of the bus, no doubt an arrangement with the bus company.
Anyway, I digress.
If you suspect you are being followed or are just plain uncomfortable, just cross the street a few times to confuse any potential followers or go inside a shop and stay close to a security guard for a few minutes browsing things. This won't shake the hardened mugger, but they might go after an easier target instead.
It is also true to say that what FEELS dangerous in a culture may not BE dangerous. For example touching others in some parts of Brazil is common among people from the same social class, and part of the communication process, although it may be perceived as a sign of danger for those who are more formal and not used to body contact.
Lastly, never trust anyone completely. New friends are great but never trust new friends with money, cameras or passports. Many thieves become friends with the victim for sometime days or weeks and then take their chance once they have gained your trust.
Sad but true.
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