Should I show up and meet her even if I'm uninvited?

OK. Husband has never before given me reason to be suspicious. Old friend (female) of his calls our house about a month ago. Daughter answers the phone, talks to the woman then my husband talks to her. Daughter says, "That's that woman who came to see him last summer. She makes me uncomfortable because she is so flirty with him." What? I didn't know she'd come to see him? The call is about her birthday party she is throwing in a few months and where can they have it. He gives her his cell and work numbers and his e-mail address so they can continue the plans. I admittedly flip out when he gets off the phone. He has no excuse for not telling me about any of this other than "It's no big deal, I didn't think of it, she's just an old friend, we never dated or anything...." I let him know that the secrecy is what bothers me, not the contact, and that in future I would appreciate him letting me know when they are in contact or making plans.