I have a pot with a roll of a few hundred pounds cash, which I add to each week. Every few months or so I go and pay it into the bank, and every now and then I pay eg the window cleaner in cash. I have been a bit lax about tracking the amount over the last 6 months but noticed back in September there was less than I thought, so I started keeping a proper log. Sure enough there have been two times when I've found less than the amount that I expected in there (most recently £60 less).
I don't think it could be anyone outside of the family, as it would take some rummaging to find the pot. I've also double checked with DH that he hasn't spent any of it. I have three kids, the eldest (16) is the most likely culprit (the other younger two don't really go out shopping or hanging out with friends. She's generally a good kid though, and I'm shocked to think she'd actually steal from me!
I'm planning on having a one to one chat with each of them, asking if they know anything about it, but I was wondering if anyone else had been through this, or has any advice about how to handle it.