How do I detect when a player has pushed a block into another block in a Sokoban-type game?

I've been following this tutorial:

construct.net/en/tutorials/push-objects-tile-based-game-2592

And so far so good. I've made it so that instead of one block, there's families of different types of blocks of different colors that all play by the same rules (i.e., they're solid when they push up against each other).

I'd like to implement things like key blocks that unlock chest blocks when you push them together. But when I try that, then weird unintentional things happen like the blocks being "pushed together" when the player is far away.

I'd also like to implement situations where something special happens when three blocks of the same color are side-by-side in a row, but I'm not sure how to make the blocks detect when something is adjacent but not overlapping.

Thank you!