Final answer:
If an open sterile tray is found unattended, it should be covered and marked as contaminated to maintain biosafety and prevent infection, then re-sterilized before future use.
Step-by-step explanation:
If an open sterile tray is found unattended, the appropriate action is to cover it and mark it as contaminated. Sterility is compromised when a sterile tray is left unattended and open to the environment, as it can easily be exposed to pathogens. The next step is to ensure it goes through the sterilization process again before it can be safely used in a medical setting. This aligns with strict biosafety practices that require contaminated materials to be handled appropriately to prevent the risk of infection and maintain a sterile field during medical procedures.