Indeed. We have enough trouble finding certain DNA-based life forms. Plenty of life forms we only know about because we leaned how to copy DNA, and started grinding up samples and amplifying the DNA. Many of those refuse to grow in petri dishes and don't cause diseases, and would no doubt be unknown to this day if they didn't contain DNA.
I think there's a fairly low chance that Earth has life that doesn't use DNA/RNA but if there is and it minds its own business, it could be decades or more before we discover them.
Consider things that grow much, much more slowly. They're already finding chemolithoautotrophs living in rock 4 km beneath the surface of the earth, that reproduce over the course of years, rather than in twenty minutes like the bacteria we're used to working with. If there were organisms that didn't have DNA, but did have some sort of body that could maintain chemical gradients, allowing it some sort of metabolism, and reproduced on the scale of centuries, we'd have trouble ever noticing it was there because we haven't made the tools to find it, for lack of knowing what we're looking for.