Car Overheating and Overflow Cap Missing

2003 Honda civic EX 51,000 miles
A couple of weeks ago, my car started overheating (white smoke coming out of the passenger side). When I opened the hood, I saw that the overflow cap was missing. I put coolant and water in the overflow tank and was able to keep the car temp down long enough to get to a dealer and replace the cap. The car drove fine for about 2 weeks. Today, it started overheating again on the highway and when I opened the hood, the overflow cap was again missing and it looked like liquid had splattered/spilled over the engine. There was also some thick gunk on the top of the radiator cap. I let the car cool down and when I tried to start it again, I had to pump the gas hard and rev the engine a little to get the car to start. Why would the overflow cap keep coming off? I don't know much about cars. Any insight you could offer would be great as I don't want to get taken advantage of when I go to have the car looked at.

Thank you