If done properly encrypted data looks like random data. That means that if you have a Raspberry Pi pumping out random data (no need for encryption), they'll probably just store it thinking it's top-secret encrypted data that they'll decrypt later.
There is the likelihood that most encryption applications would put a header of some kind on the encrypted blob.
For example, PGP, PKZip, OpenSSL, and age all do this with encrypted files. I'm not sure I can think of a user-oriented encryption application that doesn't. Disk encryption also definitely does (although in TrueCrypt volumes the header is encrypted and not recognizable as such, so TrueCrypt is an exception, probably deliberately).
Crypto primitives in programming languages don't, and NaCl crypto secretboxes don't (they just start with the nonce!), so I guess if you were calling the NaCl secretbox encryptor by hand to encrypt your files, they wouldn't have any header.
