This is an excellent question. I will find out and get back to this forum with an answer.
After speaking with R&D - the signal to noise ratio will report "infinity" if the noise value is calculated as 'zero'. You should be able to confirm this by exposing the noise column in Quant in addition to the signal to noise column. Be sure to expose many digits (perhaps 16) of the noise column (Tools -> Number Formats) to make sure there's no rounding going on. Any non-zero noise value should not evaluate to infinity.
Hope this helps.