Sure, could be. Lots of acetobacters are in the air and could form colonies with natural wild yeasts or yeasts from your kefir grains.
Kombucha is much like other vinegar mothers; it floats and contains acetobacters (and other bacteria) and yeast. (side note: claims on the net that no vinegars/MOV contain gluconic acid (or gluconic acid bacterias) are completely false -- there are several peer-reviewed studies documenting the amount of gluconic acid (and gluconacetobacters) in vinegar/mothers; traditional balsamic vinegar (TBV) mothers are particularly rich in them and it is considered a marker of quality TBV).
Some water kefir grains contain acetobactor strains and some don't. See http://www.ncbi.nlm.nih.gov/pubmed/22000549
for an analysis of some water kefir colonies.
I tend to remove any stringy growths at the top of my water kefir ferment before refeeding so don't let them get a hold. But if your new mothers look healthy -- feel free to try making straight vinegars or kombucha-style drinks with them.