How possible is it to use a lost iPhone 6s secured with a 6digit passcode?

Anyone can erase a device by restoring it in iTunes. If they did that, your data would be wiped, and your ability to track it is also gone.

But, as long as that device is listed under your device’s in your iCloud account, no one can complete the iOS activation steps to begin using it without knowing your AppleID password.

If you report the theft to your carrier, they may be able to blacklist it as well. In the USA all carriers participate in a universal blacklist. That same USA carrier-wide blacklist is now also used by most or all carriers in Canada and Mexico.

Well, if using 0 through 9 in a six digit sequence there are 1 million possible unique combinations. The iphone would be disabled after 10 failed attempts, so unless someone is extraordinarily lucky at guessing your passcode, it’s pretty unlikely they’ll get in.

(it’s literally all possible 6 digit numbers from 000000 to 999999)

If you routinely use touch or Face ID, then you could also use a complex passphrase in place of a passcode. That would make it even far less likely than a one in a million chance.

And of course, as long as it listed on your iCloud account under the tracked devices, activation lock remains in effect and keeps anyone from activating and using your device.

It is entirely possible that your iPhone is simply turned off or the battery has completely discharged. Either event would cause your device to disappear from Find My Phone. The simplest explanation is often the correct one.

The fact that the phone is coming on /off line would seem to me to be the suspected thief is trying to unlock the phone. A six digit pin code would be effectively impossible to guess and while there are ways of getting into any electronic device that sort thing is in the realm of governments and their security operations. You would have to be a high value target for that to happen. No, in my opinion someone is trying and failing to get into your iPhone. You would think common thieves would know this by now. People who bought “used” iPhones from Craigslist and other non-standard sources are always coming here and asking how to defeat activation lock. It’s simply can’t be done by the common user or thief.