first off make sure all of the tools you take are labeled or marked...that way if someone borrows it you'll know it's yours when you leave.
I use to carry tons of stuff but cut back to just a couple of shoulder totes for the 4 hour or less crops (I'll put every thing in the XXL if it's an 8 hr crop). One tote for planners, and the other one for tools I'll use.
Pre-plan as much as possible...I use the Cropper Hopper page planners...I also have decorated my planners so I know they are mine...I had two to disappear at a crop...I fill each planner with all the things I might use on the LO. I have a small box for brads and the little bulky stuff that will damage the page planner.
I really don't think people just take things to be taking them, but when we stop chatting so much and start srapping we tend to lose track of time and have to clean up rather quickly...
If you plan to go to crops often I would invest in two sets of hand tools that you use often (like adhesives, tape runners, scissors, etc...one set for the house and one set to keep in you bag...gosh it saves a lot of time packing and unpacking...You will only have to unpack your LO's and stuff you didn't use.
Good Luck and have fun!!