Small hose disconnected under hood

I just had a 50,000 check up on my car 3 weeks ago. Today I turned on the car and found the check engine light on. I popped the hood to check for any obvious items that might be wrong. I found a small clear hose with a red capped attachment sticking up in a way that's obvious that it was out of place. I can't figure out what it's should be attached to. Can anyone assist? Assuming that once it goes back to it's correct spot, the light will turn off.