You can also go to the Apple ID account website to help you find it. Go to the Apple ID account website, click “Forgot Apple ID or password?,” then follow the onscreen instructions.
Search email messages
Yes, if someone knows your Apple ID/Password, they can activate iMessage on a device & send such using your ID. However, every time iMessage is activated on a device, you'll receive a notice, from Apple, informing you of such. If you suspect someone knows your Apple ID/Password, you should change your password.
What to Know
Manage iCloud passwords on your Windows computer
Go to Settings > iCloud. At the top it will have a name and iCloud ID. Write down the icloud ID (it may be one of your non-apple email addresses) then click on that and it will ask for the for the password. If you cannot remember it, click the Forgot Apple ID or Password link in blue.
Use Find My iPhone If you have the Find My iPhone app installed on multiple devices, say the iPhone and iPad, you do not need to log in to your iCloud account to track it.
Change your iCloud (Apple ID) password
Changing the password will not affect devices already logged in (regardless which password was used to initially connect). However, once your lost ipad runs out of power, Find My iPhone will no longer help. Recharging will reset it, and that might cause a password issue. Many thanks!
To track a missing iDevice using "Find My iPhone" you can: Go to iCloud.com and sign in with your Apple ID. On another iDevice, install the "Find My iPhone" app from the app store, enter the Apple ID credentials and track it.
Apple ID password. This is the same as your iCloud password or the App Store password. This is the password for your Apple ID account, which is needed to access all Apple services like the App Store, iCloud, FaceTime, iTunes, and others.
If your iPhone backs everything up to your iCloud account, then someone can spy on your activity by accessing your iCloud account from any web browser. They would need your Apple ID username and password in order to do this, so if you know that a third party has that information, there are a few steps you should take.