The Rabbit's Foot was the last sample of the "Chimera" virus from the previous movie.
There is no way a lab would create a powerful virus like that and keep the only samples of it in one place, I speculate that Biocyte went down after the events of 2 and someone else bought up the storage inadvertantly coming across a single sample of Chimera, after all, it was shown in the previous movie that after 20 hours not only does the victim become infectious, but the Chimera virus becomes uncurable, it sounds plenty like something a sociopathic arms dealer like Davian would want, or... The Rabbit's Foot was a mole hunt/decoy
Like the NOC list at the start of the first film, designed to draw out people like Davian into the open, so the IMF could get to them easier, to an arms dealer, a WMD of that kind of rumoured power, is basically catnip for them. the Rabbits foot was just a container with nothing in it, except maybe a tracking device once the seal was broken. and like the first film, they never bothered to tell Ethan that it was faked, and they never expected Davian to go so far trying to get it.The Rabbit's Foot is the Luck Virus of Red Dwarf
The Red Dwarf episode "Quarantine" introduced the concept of positive viruses, with the crew being particularly keen in the sexual magnetism and luck viruses; the first makes the subject temporarily irresistible to the opposite sex and the second allows the person to achieve basically anything they want out of pure luck. The sheer potential power of the luck virus would make whoever owned it basically unstoppable so long as they could use it the right way to maximise their few moments of luck. Lindsey Farris had a crush on Ethan
