I have a confession to make, which when it prefaces my answer, will make some sense to you. I like fringe science theories. Mind you, I don't necessarily believe them (I struggle to think of any that are plausible), but they help me to understand the border between plausibility and implausibility.
One such theory that I read about years ago posited a sort of single-celled life on earth, the diameters of which fall below the minimums for single-celled organisms. At the time there were one or two popular articles on it, rainwater had been falling someplace and was discolored (red I think?). Some fringer suddenly appeared and was claiming that when observed under a microscope, small red particles were observed and were a novel kind of life. The particles were maybe 50nm in size (this is more of a guess than a memory).
No previously discovered single-celled organisms have been discovered that were so small, and to date none have been. Imaging these things would be difficult to determine if they were indeed alive, and more importantly, it seemed impossible to culture them. That last one's important, it's impossible to culture most singled-celled organisms successfully (and yet we're certain they're alive).
The fringers like latch on to things like that. To find little places to with enough wiggle room that what they claim isn't impossible. And indeed, it's not impossible that this was an incident involving a novel life form. The minimum size of a cell is indeed set at 150-200nm because knowing what we know of life, there is some minimum equipment necessary for one to be alive and be able to reproduce, and it doesn't seem to fit in less space than that.
But if it wasn't using the normal equipment, all bets are off. It could use another molecule to encode genes. It could use different amino acids, different base pairs, different codons... codons that use more than or fewer than the triplets of base pairs. These could use left-handed vs. right-handed amino acids. They might not have ribosomes like we have (the little robots that take a strand of RNA and use it to build a specific protein).
And, not only might this reduce the size of the cell (some of them anyway), it would make it nearly undetectable to some of the very processes and equipment we use to detect life. Especially if no one is looking for those specific things.
Such an organism could slip under our radar, and might be doing so even now. And as far as life goes, we aren't even yet discussing anything especially exotic. If life can exist in what we consider hostile environments, than anything in the deep crust or upper mantle is completely invisible to us (currently). There could exist another wholly independent tree of life in the deep oceans. Or the upper atmosphere. If these things are microscopic and are not antagonistic to our tree of life, then they might even be living in that dust bunny on the desk behind your monitor, right now. Slow-growing, always present, small enough biological (if that's even the right word) mass as to go unnoticed.
I have no reason to believe such an organism exists, and more than a few to believe that it does not, but it wouldn't be impossible. We might miss such things, even when looking directly at them. Might not even believe them to be alive.