My rear tyre (8 month old S40 - 7000ks, lots of tread left) was way down in pressure when I checked it - 20 psi instead of 33. (It didn't feel right at all when riding) So I filled it but the next day it was down even more - 18. (After a 20 k ride to work and back) So of course I thought a leak somewhere - will need repair, replace.

Cos I have a foot pump at home I put it back up to 33 to see how much it went down overnight. I noticed that the little bolt thingy (at base of valve where it touches rim) was loose to I tightened it up at the same time.
But it didn't go down again, and 2 days later and 40 k riding still hasn't lost any pressure.

Any idea what could have caused this? Could it have been the  loose bolt - whatever it is called (not the dust cap) I would have thought once I had a leak then it would not just go away. Should I just keep riding and check every day?
Most of my friends with bikes have tubeless tyres, so they don't really know much about running tubed.

I will be going away  in a couple of days to work on an
Army Base so the bike won't be ridden for 7-9 days. If I have a real slow leak maybe it will show up then ??

duh - "bolt thingy" I meant the nut at the base of the valve stem was loose.  

For future reference:

Re valve cores, it's not a bad idea to remove them and dab tiny amounts of silicone grease on the rubber surfaces.  You will see two different seals you can apply the grease to.

I ride about 40K miles per year on my bikes and tires are my number one mechanical concern.