Sideway: New York

Each non-boss level has 5 Secret Tags and up to 3 Unlocks (Powers and Upgrades). You can see how many you collected for each on the level select map; just move over that level and wait a moment for the status window to show. While playing any level, you can also hit to see how many you have/need. You can quit after you get them, or restart the level freely, as it saves either way (despite the game stating all progress will be lost).

You can keep replaying each level till you collected all the booty. Some of these will not be possible to reach till you have a certain power, which may be on another level. It is strongly advised to collect all the upgrades first, then go for the secret tags. Many levels have a cloud-? Graffiti that triggers a clock and a bunch of score tags - collecting all these score tags before the clock timer ends, will allow you to snatch a secret tag (if you haven't collected it yet). Once you collect a secret tag or power up, it will no longer appear on the level.

Each level also has hundreds of score tags, which you (thankfully) will NOT need for this trophy! Just collect the 55 secret tags (5 secret tags on each of the 11 non-boss levels), and this trophy will unlock.

Thanks to FrankNStein for these great videos: