Wasn’t Snape the one who alerted the Order at the end of OoTP. How is it that no one else recognized his Patronus? Sirius would have had a field day with that one.
Sirius and Lupin probably did recognize his patronus, but it would have come as no surprise that he loved Lily Evans -- they knew that already, at school.
And perhaps one of the reasons Snape hated Sirius so much was precisely that he would recognize his patronus.
We don't know Snape's method of contacting the Order then, do we?
I was convinced that the doe Patronus was from a girl. Fooled me with that.