I do highly recommend going secure with your cannisters. Some folks love pinching ore even in high sec just to mess with people (I see it every night; you're lucky not to have run into it yet!). There are plenty of folks out there who are set up for pvp and don't want to deal with mining but need the minerals, so they just poach and blow you away if you try to take it back. Or warp out really fast.

Okay, confession time, I know this because there are some folks in my corp who take a roguish delight in pulling off all sorts of scams, so I hear this stuff all the time over vent. In their defense, they aren't total twits---often they purposely go after the cannisters that are obviously put out as bait rather than lowbie cannisters because, of course, that's more of a challenge and thus more fun. And if a miner is a good sport they'll generally leave him alone as soon as they've had a little fun, and move on to other pastures. :)