Something is on the LAN that is being sent to the printer to be printed, probably not intentionally.
Wireshark is the only method to root out what and where.
Easier method is to simply change the IP of the printer, which will at least break anything sending to the IP of the printer, but won't stop something sending to its host name.
There are virus's that do that
There are hackers who do so to people who think it a good idea to port forward to the printer.
Security software running on the LAN also occasionally try and telnet/ftp/smb submit data to the printer to expose exploits, and a printer not being a computer, just prints garbage because it can't do anything else, it isn't capable of opening scripts.
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