Of course, there is a proper, scientific explanation to this (I doubt the rabies virus thrives in sulphuric acid and lead). That is that the highly contaminated, cracked cell is leaking a lot of current and is being discharged by only sitting. When you discharge a lead-acid battery, you essentially combine the acid in the electrolyte with the lead on the plates, creating lead sulphate. It normally collects evenly along the sides of the plates, but since a wall is missing, nothing prevents it from forming on the edge of the plates as well, growing up in the air.
I didn’t know that lead-acid batteries were susceptible to rabies, but the cracked battery I found in an APC UPS a while ago certainly seems to know better! After sitting untouched on my bench for about a week, it’s without doubt enlightened me on the subject!