If you haven't hit anything and you haven't parked your bike on a busy bike rack (other tires getting accidentally rammed into your spokes), the most likely culprit is a nipple vibrating loose.
You can see if this is a likely possibility fairly easily...one, maybe two, of your spokes will be noticeably loose...less tension then the surrounding spokes (compare to spokes on same side if a rear tire...can be different tension to each side).
If this is in fact the case, it's something to be a little bit worried about in that you shouldn't just true the wheel and go on. If a nipple is working itself loose you will need to do something to keep it from doing so again. Generally a nipple will only work loose if it's damaged or if there wasn't spoke prep (similar to loctite) on the spoke when it was built.
For damage you'll notice a crack upon close inspection or it may be "squashed"...no longer square...this happens when someone bottoms out the nipple while building it or in some cases of too much tension...this out of square can quickly lead to the nipple cracking. If this is the case, remove the nipple and replace. Retrue, check round and dish and get back on the road.
If it's just loose, you're (MOSTLY - see below) in luck as it's the easier fix. Loosen the spoke even more, add some STUFF (see below) to the spoke threads and then retrue, check round and dish and get back on the road.
STUFF - You need to put something on your spoke's threads to keep the nipple from working loose under vibration. The purists will claim that only spoke prep should be used and that loctite will ruin your wheel. I disagree. I've built several sets of wheels when I didn't have spoke prep around and fixed many more and I'm happing using blue loctite (NOT RED). If you can find it green loctite is probably an even better choice. If you have money to burn, go ahead and pick up some spoke prep...it's good stuff, just pricey for someone who is not getting paid to build wheels. If you use blue loctite, just a tiny bit will do it. If you use spoke prep, make sure to let it dry a bit before tightening.
MOSTLY - I said that you were mostly in luck. The downside is that if there was no spoke prep on this spoke, there may not be any on other spokes. You'll want to check it. Some spoke prep has a bit of color and you can back a nipple a couple of turns and see (barely) a bit of blue or orange. Loosing a spoke during a fast decent can be very dangerous so if in any doubt take the wheel to someone who builds them and have it checked.
