Toilet/pipe Problem?
I can think of three possible explanations:


The wax ring under your toilet just wore out and needs to be replaced. This seems unlikely if your house is only 16 years old; a properly installed wax ring should last 20-30 years.
Your toilet is not seated securely. This can happen if the bolts are not tight. If you can rock the toilet at all, this is probably your problem. When the toilet gets loose, the seal with the wax ring breaks. In this case you'd want to remove the toilet, fix the wax ring (or replace it with a new one) and then reinstall the toilet, being sure to tighten the bolts all the way but not overtighten.
At some point water pooled on your bathroom floor. (There could be lots of reasons for this: Someone took a shower with the curtain open, the faucet overflowed, someone tipped over a bucket, etc.) The water found its way down through the toilet flange hole. It picked up dirt along the way, hence the yellowish color.


Based on the information available, it's hard to say which of these three scenarios, if any, is the cause. If I were you, I wouldn't do anything at this point. I would keep observing: Go down in your basement once a week and see if you see (or smell) anything new near the stain. If you don't see signs of change, this was probably a one-time event, like water pooling on the floor, that you don't need to worry about again.

I'd also check to see if there still is water pooling on the bathroom floor. Check especially behind and alongside the toilet; it's possible the toilet tank is leaking, or the water supply line is leaking. But if you can't feel any water anywhere on your floor, then you most likely don't have an active problem.

You could replace the wax ring if you want. It's cheap (wax rings cost under 3 dollars) and relatively easy. But if I were you I wouldn't bother at this point because -- unless the toilet is loose -- I doubt the wax ring is the problem.

For what it's worth, I can't think of anything that could be wrong here that would result in catastrophic damage. Worst case, you have a small leak in your drain system somewhere, you caught it early and it would have to be left unchecked for months or years before it caused real damage. Best case, this was caused by a one-time event in the past that will never recur.

If you were suspecting leaking from a water supply line, I'd tell you to be more worried, because if that were to burst it could do significant damage very quickly. Not so with a drain line in this type of situation. The fact that the leak is over unfinished basement makes it even less concerning.