Missing Device

If the device has not been online for more than 24 hours, the last location drops off the radar.

See this link about what to do if your phone is lost or stolen --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If your device is powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple, you can't see your device's location. Learn how you can still protect your information. For more info, see:

Use the Find My app to locate a missing device or item - Apple Support

To locate your iPhone in the Find My [Device] app, you must have activated the Find My app BEFOREthe iPhone was lost. Here is some information on that app ➔ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.

Note that your stolen iPhone 6s Plus must be able to make the connection to the internet via Wi-Fi or cellular connection so that it can mark its location and broadcast it. 

Axel F.

That's correct. If it's offline and not connected to a network, its location cannot be seen. Be cautious about bad actors who may send you an email appearing to come from Apple, asking for your Apple ID and Password. This is common practice from thieves. If you give them what they want, they will indeed have what they need to erase the phone and set it up for their use. Apple does NOT EVER send such notifications.

What do you mean by "flashes" the phone?

If the phone is activation locked, the person who has your phone cannot wipe it to use it. Activation Lock cannot be bypassed, so they can't flash it as you have said.

You're welcome and good luck.