As above, nothing to worry about. Although most say it is caused by condensation not being vented off due to short runs, my personal experience was different. About 3 years ago I got exactly the same issue but my daily motorway commute of 60 miles should have meant I didn't get this problem. I'd clean the cap once a week but every time I checked it again it had come back. It wasn't until the warmer weather came that it stopped doing it. Strangely I never had the same problem in following winters. I put that down to it having been serviced and fresh oil.
What I'm trying to get at is, yes it's nothing to worry about but don't expect it to disappear forevermore if you go for a longer run - you might just have to wait for summer!
