About "how trusting should I be", I would like to quickly bring up a bit of the scientific research. I've read a bit of this, and have concluded in high certainty that individual trust gives off a decent positive externality.

Quite a bit of econometric research (yea, it's not the best, but it's what we have) shows that positive levels of trust correlate with higher economic growth.
See: http://oep.oxfordjournals.org/content/56/1/118.short http://onlinelibrary.wiley.com/doi/10.1111/1468-0297.00609/abstract

However, I very much remember reading about how entrepreneurs have lower trust levels than most people on average, but can't seem to find the paper right now.

What I'd imagine is that in general trust helps society, which makes a lot of sense as it allows trade to function. However, it may very well be that the level of trust optimal for an individual is quite lower than that optimal for his or her society. Makes sense if trust is looked at similar to altruism.

In this case, obviously the more trust people have with other people and their bags, the more cases of stealing that will occur. On the whole this may help your surrounding people, as some of these will benefit from stealing. However, the disadvantage would come to yourself at the expected cost of having your stuff stolen.

It would be interesting to factor in not only how much your bags being lost would cost you, but how much they would benefit the person who steals them.