I have not taken all 13 cats (plus 1 foster) to the vet. The one cat that I actually CAUGHT in the act, did go. His urinalysis was A-OK but they put him on antibiotics for a fever. He went through the medication, his temp. is back to normal, but he did it again. If I have more than 1 doing it, I have not caught any one else.
Since I DO have a new foster, running around with the other cats (and this is the first time I brought a new cat in during the time that Jim..the culprit..has been living here) That is the basis for my concern that it is behavioral.