Apple - Can one's iMessage/FaceTime identities be hijacked at a public Wi-Fi?

Yes, this may be a security risk. But it's unlikely that someone hijacked your Mac using Wi-Fi alone.

In order for you to receive this message someone needs to login with your Apple ID credentials with a device that supports FaceTime (iPhone, iPad, iPod touch, any Mac).

---

Change your Apple ID password

1. Go to Manage Apple ID
2. Log in with your Apple ID
3. Go to Security > "Change Password..."
4. Change your password
5. Also turn on two-factor authentication if it isn't already

---

Check your Devices

1. Go to Manage Apple ID
2. Log in with your Apple ID
3. Scroll down to My Devices
4. Click any device that looks unfamiliar to you
5. In the new window click the blue circle with an x to remove it.

---

You may need to sign in with your new Apple ID password on your other devices in order to use all iCloud services again.