I buy tinned pineapple chunks, and then I paint them black. Normally with shoe polish. Then, once the pineapple chunks are dry (which doesn't take as long as you'd think, as long as you follow the instructions on the polish i.e. dry the chunks before painting, in an airing cupboard or on a south facing windowsill, removed from juice), I put them back in the tin (sans-juice), reseal the tin and take them back to the shop.
I won't buy the shoe polish and the pineapple chunks in the same shop. No reason, really, but I do like to use new shoe polish each time, and it's best not to give them any way to establish a causal connection.
Once they are back on the shelf (it is actually quite easy to tell a resealed tin from a factory sealed tin, even though my equipment is catering grade) I tend to loiter around the tinned fruit section for a while, reposition my tin at the front of the stack, label first. I take a picture on my mobile phone, then email a copy to myself and erase the original photo. If no one has picked up my tin within the first half an hour, then I have to leave. I've established the amount of time you can stand in one section of a supermarket without the security starting to get suspicious; half an hour is definitely the upper limit. Normally, I'll take a walk around town, and pop in on my way back, just to check that it is still there. Often, if I've positioned it well, it will be gone. If so, I can go home and make a note of it on my Excel document. If it isn't, then I will return the next day, and on consecutive days, but only to check. I can loiter for half an hour, without the instantaneous appearance of a security guard, about every one and a half weeks (because of shift patterns, and security hand-over briefings and possible print outs from CCTV cameras of certain persons faces, and available space for said print outs on the security guard notice board in the little office with all the televisions).
I have only had a confirmed hit i.e. a viewed (by me) purchase, twice in the time that I have been doing this properly i.e. not just for a laugh, but seriously, with the spreadsheet and distribution pattern. Fairly early on, once in Morrison's and once in Waitrose. In Morrison's I found the resealed tin and stood there for two or three minutes max before someone came along and put it in their basket. I followed them to the tills, picked up some chewing gum and a copy of Take a Break (for Denise, but also by accident, and also because it had Tamzin Outhwaite on the cover, and I like Tamzin Outhwaite, always have.), and watched them pack the tin in to their bags. There was a problem at the tills once the lady had left, with the tin, and I had to wait for the girl behind the till so I didn't manage to follow the lady out of the shop. Once I had got to the exit, she had gone. Probably on the bus, or in a taxi. A lot of the poorer people get taxis from Morrison's, which I always find fascinating.
The other time, as I mentioned, was at Waitrose. I had loitered for my half an hour on the first day, come back for three days in a row for a check, and on the fourth day I arrived at the tinned fruit section (quite small in Waitrose) and thought the tin had gone. I moved a few tins around to look for mine (often it can be shuffled to the back by an overzealous assistant, rotating stock by sell by date) but it wasn't there. I looked down, and clocked the tin (the resealing is really quite obvious, if you know what you are looking for, but obviously, most people are looking at one tin at a time, and even if they are not, shop workers are either not bothered if they notice, or can't be bothered with the hassle of trying to remove stock from the shop floor and pointing out what is - once you take it up to middle management - quite an insignificant aesthetic problem) in the near empty basket of a man, about my height and build. He walked towards the checkout and I followed discreetly from behind. At the tills, I picked up some chewing gum and a paper (no Take a Breaks in Waitrose, well, not by the tills anyway) and put down a little plastic separator, to separate his shopping from my incidental purchases. I allowed myself a quick look at his face. He was looking about the shop, glancing at other customers and trying not to smile. I looked back down at the conveyor belt and saw three tins of pineapple chunks and a small round tin of black shoe polish.