Marriott's website should be able to show you exactly where the 500 pts came from. Just log in and navagate to your Marriott Rewards Activity webpage. From there, either the description will tell you what it is, or if it is from a specific hotel, you might see a little sideways pointing triangle. If you left-click the triangle, it will point downwards and expand the entry to show you all the details from the entry. If the entry is your Plat arrival bonus, it will tell you that.