I have the screen timeout set to 30 seconds, and its set to lock 5 seconds after that.
I tried "unlocking" in this 5 second buffer when the screen is off. Tapping anywhere on the black screen will immediately spring it back to life (as expected). If I instead happen to use my fingerprint on the sensor, the same thing occurs, but it shows the fingerprint animation (white light under the sensor location) , which makes it appear as though it accepted my fingerprint and "unlocked" but in reality its just waking the device back up before it locks in that 5 second window.
It's possible that this is what I've observed on these 4 occassions - I catch the device in that 5 second buffer where the screen is off, but it hasnt locked itself yet. When I go to "unlock" my phone with fingerprint as is muscle memory, it simply wakes the device back up, while making it appear as though it accepted a finger print in the process.
With this behaviour in mind, I'll keep a close eye on it to confirm if there are any instances of true unlock without a PIN.