I recently (one week ago) left my Fairphone 3 in the Flixbuss from Brussels to (Hamburg) it had lots of stickers on it and on the battery itself too. Also had a black bumper on it. I filled out the lost and found form from Flixbuss but so far no luck! If anyone comes across it Please let me know!
If you can still find the IMEI number(s) (e.g. from the package the FP3 came in), please contact Fairphone Support to submit these to them. They are keeping a list of Fairphones reported as lost or stolen so people buying Fairphones second hand can check if an offer is legitimate. I know it doesn’t really help you, but it could help others.
If your phone is connected to a Google account, you can try to locate it using find my phone. You can also lock your device with a password and add a message to the lock screen, good luck
If you aren’t running the stock rom you might want look into whether whichever rom you are on provides a similar service.