Leave it to Google to make April Fools fun long after the day is done. Users who successfully completed the Maps quest and found all the Pokémon (plus the secret Mew) are being pinged by the search giant with a link to register for a surprise gift. What is it? Well, that's what surprise means.
The link (redacted above) goes to a Google Drive form where you add your information and verify your achievement by including the location the final secret Mew was found. It only showed up after all other 150 were captured, so you'd need to have finished to know the answer.
Google will take 4-6 weeks to ship out the mysterious gifts, so it's definitely a physical thing headed to your door. Let us know what it is, if you managed to snag 'em all.
[Thanks, +Luan Cristian da Silva]