Was it perhaps unlocked when you lost it?

I know I’ve set my phone down unlocked a few times; particularly at work (in a warehouse).

Is your pin something like 1234? Do you have emergency contacts set up? Do you have a setting to not lock the phone until very long? Or a smart unlock based on location or any other automation setting? An easy password hint pops up or something? Perhaps your parents forgot to mention you had a twin, who face unlocked it.

Regarding app usage, my guess is they tried to see whom to contact to give your phone back, or map history, the banking app could be a touch by mistake too.

Do those contacts happen to be your ICE? Some phones will allow those from the emergency dialer without unlocking. Don’t know about grapheme.

They clearly seemed to mean well. Maybe you can ask?

I imagine you may have lost your phone while it was still unlocked. It’s possible that there’s a Graphene lock screen bypass out there, but I doubt someone with such knowledge will use it to return your phone to you. Most “hacker” style lock screen bypass I imagine someone wanting to return the phone will do is checking for smudges on the PIN area of the lock screen and determining the code from that.

To combat someone unlocking your phone through smudges, you can enable PIN scrambling.

This maybe a strange suggestion. Aside from the banking app, it seems like the maps and contacts app were used with good intentions to return the phone. The person returned the phone to your friend, so clearly had good intentions. Your friend may have the phone number of the person in their call log when they called to return, unless of course they used your phone to call. If possible, have you thought about calling that person and asking about this just out of curiosity?

When you find out let me know

Perhaps they simply took out the sim card and inserted into another phone, giving them access to contacts (that could have been saved into the chip instead of the original phone)?

If someone calls you and theres a missed call notification can they just click it to call back without unlocking the phone?

Oh i didnt notice they went through other apps. Maybe they were watching you and saw you input your pin and then stole it and checked your stuff to see if they can get something useful and then returned it?

Is it possible that something else was installed to the phone? If they manage to hack it open, then potential reason to return it to you is to spy on you.

Unrelated. Have you considered using a work profile?

That way you can have two factor authentication when unlocking your phone. You could use a PIN code for the main unlock, and a biometric for apps in the work profile. That way you would have to have both something you are, and something you know.

What I said, a smartphone can never be a secure device and this is why it is crazy to have sensitive data stored in these gadgets. It is certainly unlikely that this person gained access by trying the pins, because the cell phone would be blocked after the third failed attempt, but even so, a computer expert needs seconds to access, bypassing this little protection that cell phones have. You can be happy that your Secure Banking app is better protected, probably with 2FA, coordinate card, ID card or similar. In any case, this mobile phone is already compromised, which makes it necessary to change at least the credentials, better even your phone number (ask your ISP). Also be attentive, since this person may have been honest, but he returned it may also be because he was not interested in the cell phone, but in the address where you live.

If they were able to guess your pin, you should probably switch to a longer pin or a password. It seems insane at first to type a long password, but if you pick two long scientific words, its secure with only letters.