They can decrypt it as it appears elsewhere, then see that your node has the same cyphertext.

Example: Am cop, run "Watch bad movies off the blockchain" app (which decrypts cyphertext and plays the plaintext movie file). Find bad movie, see that it is bad, note the cyphertext. Still am cop, come to your house to look at your node, see cyphertext, confirm that it is identical to "bad move" cyphertext - arrest.