When you opened a box of Cracker Jack™ (the product jingle 🎶 went "candy-coated popcorn, peanuts, and a prize, that's what you get with Cracker Jack") you never knew what little prize you were going to get. It would be some small charm, ring, or other trinket.
He does not know what kind of situation he will encounter. Life among the rough sleepers (a British-English euphemism for the homeless) is unpredictable. "Mark never knows" implies that Mark has visited the tent city multiple times, and has been met with a variety of challenging situations. There is an element of surprise, but not pleasant surprise, since "what he is going to find" (a collocation) has somewhat ominous overtones.
Sometimes on my way home from work my heart starts beating rapidly.I never know what I'm going to find. Last week the dog had swallowed a mitten. A month ago our twin boys aged 9 started a small fire in the kitchen trying to toast marshmallows over the gas stove in the kitchen. And this past summer they were squirting each other with the hose in the back yard but never turned it off, and I didn't discover it for three days. You should have seen our water bill!