I have the OfficialGameGuide and I've done everything it said, and I still missed one mini-kit. The arrow points above a tree by the park. After damaging the Joker robot for the first time, Two-Face's jeep is suppose to appear again so you can destroy him and get another mini-kit, but he's not respawning for me. This is the one I think I've yet to get.
If it is the cars then here is what I ended up doing, hope it helps: From the beginning, immediately aim for the feet so that you target the first car and destroy it. Once it is destroyed, swing the reticule to the right so that you can target the first silver strut you have to destroy. If you miss this first strut, do not go after the others! Try to remember where it was and then when it comes around again get it then. Once you do, then the other car should appear and if you target at the bottom of the Gundam you should lock onto the car and take it out.
The reason for not going after any of the other struts is that, I think, the roads technically change once you destroy the struts and the second car is going on a predetermined path that gets screwed up if you shoot the others. If you shoot the second strut before the first then the second car gets lost behind Joker's feet and then goes off a side street.
Aside from just driving around with the reticule to the left or right a couple of times, you might be missing the helicopter that is around the bridge area on the right side. Not sure when this was available, though. There was someone else having issues with the purple canisters, namely the one in the tree. Other than the cars, the helicopter and the one canister in the tree, I can't think of anything else.
That's great! I got most of the canisters my first run through so I must have gotten that one right off and didn't notice it. When I went back all I had to do was the Joker cars about 5 times to get it.