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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We have a Xerox 7500 and everytime we turn it on, it always wants to print 69 pages of one to a few lines of random gibberish text. I've noticed this only happens when it is turned on and connected to the network. I've check the print queue and there isn't anything waiting to print.
I've check the PCL settings and thats all turned off.