Cockney rhyming slang for a thief, because tea leaf rhymes with thief. Often said with a knowing look when you reckon someone’s nicking things or looks a bit sticky-fingered. It can be a proper insult, but it’s also used jokingly between mates when someone pinches a chip or nicks your lighter.
"Keep your wallet zipped on the Tube, bruv. I blinked and some tea leaf was already elbowing out the doors like nothing happened."