Because she mailed me.
How can I tell if my wife has a secret email account?
Ask her. Sneaking around hasn't work for you.
You obviously have a reason to think she has a secret account unless you're just a jealous husband.
Ask her, tell her why you feel this way. Open the door for her to be honest with you.
Keep nosing around in her stuff. If she doesn't have one she will get one. My guess is she will use it to contact a divorce lawyer.