You can tell if some stuff is tampered with - stuff in jars comes with a seal that tells you if it's already been opened, bottles have the plastic ring attached tot the lid that separates when it's opened first time, with things like cans and yogurt it's also obvious if it's been opened beforehand. IDK about other stuff but things like pasta/rice and veggies seem really hard to tamper with before cooking. Once the others come back, if it starts up again, and until you can figure out who, stick to stuff you know hasn't been touched. And maybe offer to do the cooking...
Although it might also be worth getting an allergy test done. As someone who's allergic to pepper, I probably got sick 2 dozen times before I realised that one of my friends thought that by "allergy" I meant "fussy eater" (once he realised he apologised but like that was... a thing that happened)