Lost cap on my hydro tank...how bad is this?

The guys bring a mower back after a day of mowing, and the cap is missing from the hydro oil reservoir, and some hydro oil is splashed around. There are clippings and dirt stuck to the threads and top of the filler neck. Clearly, debris was floating all around the open filler neck. They have no idea how long the cap was missing.

Anyhow, I have a new cap, and new hydro oil and filter is easy enough to pick up. I assume I should do a hydro oil change.

Any suggestions on how to try to stop...

Lost cap on my hydro tank...how bad is this?