Today, there are rumors about an upcoming announcement of the discovery of (non-conclusive) evidence for life on Venus, apparently they detected some phosphorus compound that is not produced by any known abiotic chemistry, but is produced by terrestrial bacteria.
Of course, the more parsimonious explanation is that there is some abiotic chemistry that produces this phosphorus compound, and we just don't know of it.
It occurs to me that this is not the first time this happened: the Viking landers' labeled release experiment on Mars also gave a positive result, and as I understand it, some of the people involved maintain to this day that it actually detected life. Again, the standard explanation involves a hypothetical abiotic process.
One might tell a similar tale about the fossilized bacteria in meteorites from Mars.
Now, I do not doubt that in most or all cases the abiotic explanation is correct. But there seems to be a pattern here. How often has this happened? How many more examples are there of claimed signs of life in space being dismissed as being caused by poorly understood abiotic processes?