not bugs evolved in an alien environment we don't fully understand. Even then, specialized techniques (such as staining) are required to ID even common terrestrial species. Not to mention, many microbes when placed in a hostile medium (like a Terran spacecraft incubator), become inanimate spores and wait for conditions to improve before they restore themselves to active states. These spores usually look like small, featureless spheres under magnification, and cannot be positively distinguished from non-biological structures. Remember those "bacteria" which were supposedly identified in a Martian-origin meteorite? As I recall, they are still being argued about by bacteriologists. Microscopy is intricate, delicate work, even when performed on familiar critters under comfortable and controlled lab conditions.

Even the chemical tests used to identify biological activity gave conflicting results on the Mars landers. The presence of life could neither be confirmed or dismissed and are still being debated in the literature.

Single celled creatures are usually not mobile, active organisms, they are little floating chemical factories that interact with their environment through their cell membranes. There are exceptions, amoeba pseudopods are one I can think of, and others have cilia and flagella, but not all. And we have no way of knowing if Titanians have evolved such organs. Even a highly mobile colonial multicellular organism like a slime mold would not be detectable from a microscope snapshot, you would need a movie--and then only if you were lucky enough to catch it in its walkabout mode.

Of course, anything moving around is almost certainly alive, but if they are stationary, we can't tell by just looking at a picture if they are biological or chemical or geological. Besides, any alien life we place in a terrestrial instrument will almost certainly recognize the surroundings as hostile and either die or go into protective hibernation.

As for the idea that NASA is masterminding some sinister plot just to ensure its future funding, I find that absurd. Even your example of the 21 year gap between Viking and Pathfinder seems to suggest just the opposite. This alleged conspiracy to avoid life search had just the contrary result: it discouraged further missions.

I agree that sometimes individual strategies can be influenced by perceived desires or imagined benefits; things like confirmation bias do occur even among the most distinguished and honest researchers. But to think that NASA is deliberately planning its programs that way is inconceivable. I would really like to see for myself that source which you claim supports this charge.