"Sneaky suspicion" refers to a feeling or hunch that something is true or may happen, but you're not completely sure. It is usually a suspicion or doubt that arises secretly or slowly without any clear evidence. Full definition
We have a sneaky suspicion that it might be worth more than we have accounted for. (cheesyfinance.nl)
Or maybe my mom had a sneaky suspicion that if she made a casserole topped with tater tots we'd never want anything else for dinner. (amandascookin.com)
I have a sneaky suspicion that with so much attention on Gronk, the Patriots are going to feature Cooks a lot this Sunday. (sbnation.com)