"The blood inside her locker could be from anything, first of all."

Just putting that out there, before anyone starts panicking. Without speaking to this Elena woman he doesn't know what state she was in before, what blood deprivation does to her or her kin, what mixing sources might do to her, and so on. Severus isn't exactly a vampire expert and he's not so naive to think that even what he knows of at home will apply seamlessly here. There will be no jumping to conclusions in any direction.

"Did she actually bite you or did you cut yourself and let her suck it out without her fangs penetrating you anywhere?"

There are a lot of people who wouldn't be able to say that with a straight face. Severus Snape has many skills.