This actually happens more often than people think, and banks usually don’t catch fake checks right away. The funds can appear available before the check fully clears, which is where people get into trouble if they spend it too fast. I read https://hollywoodworth.net/news/what-if-someone-deposited-a-fake-check-into-my-account/
and it explains that even if you didn’t deposit it yourself, the bank can reverse the money later and you could be responsible for any negative balance. From what I’ve seen, the safest thing is to contact your bank immediately, explain that you don’t recognize the deposit, and let them flag it before it causes fees or bigger issues. Waiting it out usually makes things worse.