You probably shouldn't, but if you must, here are some best practices
Food seems to taste better when it belongs to somebody else. When you live with a roommate, that’s especially true. The rule that “If it's in the house I'm going to eat it” is painfully obvious with a roommate's food. It's like being in a hotel room with a mini bar, except you can totally skip out on the tab. According to the USDA, the average distance between American families and their nearest grocery store is 2.14 miles, but the average distance to your roommate's food is only about 10 to 15 feet. It's so convenient, and regularly begins a clash between responsibility and immediate gratification, one where responsibility doesn’t always win out.
Besides, stealing a roommate's food is an efficient way to expand your culinary horizons. I've learned so much from theft! Greek yogurt, kimchi, okra, Bugles, those frozen Trader Joe's Pesto Linguine things—none would be part of my regular diet had I not betrayed my roommate's trust and eaten them while he was out. Should I be deprived of growth and education because those foods didn't belong to me? Yes? Alright whatever.
When stealing, one must first distinguish between low-priority food and high-priority food, which all comes to down to location. You need to focus on foods that can be grazed upon without immediate discovery, the low-visibility and long forgotten items at the back of the refrigerator or cupboard, like cottage cheese, orzo, hot chocolate powder, popcorn, beans, oatmeal, and canned soup. Who knows what happens back there?
It's the delicious, touristy foods at the front of the shelves which are tough. Some are unstealable: Ferrero Rocher, your roommate’s carefully prepared lunches, ice cream sandwiches, pizza, pudding cups, chocolate milk, entire pineapples, hot dogs, and eggs. No one forgets how many eggs they've used.
With meat, bread, and cheese, tact is required. When your roommate discovers their food missing, it's essential for them to believe it was their fault, and that they forgot how much they ate, without suspecting another party. You're probably good at this if you've ever framed someone for murder.
The angle and state of the food must appear untouched. If the block of white cheddar had a jagged edge and you've eaten two inches of it, recreate the jagged edge. Smooth away guilty holes in ice cream, and slide over cookies to the open torn part so the sleeve appears full. Always pick at the underside of any roast or turkey carcass, and roll up bags of chips to the precise curvature you found them at, even if there's only one chip left. Your goal is to avoid discovery, or at least delay it, so you can later replace the items when you feel the heat around the corner.
Steal all the roommate tea you want. The kind of person who has a tea collection is also the kind of person who won't do anything about stolen tea. The most you'll get is some sort of passive-aggressive remark, like “So, have you been enjoying that oolong tea?” You can even stare at them while steeping, and sling the used tea bag at their door, where it will stick and slowly slide down in a taunting fashion. I feel like I'm losing people.
If you’re trying to avoid roommate food theft, you have a few options. Angry notes or a mini fridge with a padlock are extreme, but there’s a simpler solution: Since most illicit grazing relies on the food being already opened or partially eaten, leave the fridge full of delicious, sealed food, and don't touch it for weeks, eating all your meals outside the house. Your roommate will anxiously wait for the lids and seals to be breached, and then one day, when they can take no more, you'll find them sprawled out against the open fridge, covered in your food, and tearfully screaming, “I'm sorry, I'm so sorry!”
“That's okay,” you can say, petting their head. “Let's go to the grocery store together.”