I have 5 year old house. I found dried dirty scum all over sink. I am not sure where or why it came out. It is a bathroom that is rarely used. Any thoughts?

Is that 2 story house ?

No it is a one story! I am very perplexed by this situation!

This "scum"/ Is it a one time thing or does your lavatory back up often? Does the lavatory drain? Please give us more details. Tom

The "scum in bathroom sink was a one-time occurrence. It was as if the the sink "sprayed" dirty water all over basin. The sink drains pretty good. It may have a slow leak but the sink stopper is difficult to remove so not sure how to check. It seems to be hooked in very tightly. The toilet rarely back ups but has in the past. I haven't had to use a plunger in the last few months.

If you need more details please let me know as I don't know what else to share.

Thanks again for helping me with this situation.

IT sounds like the line was pressurized and simply blew gunk up out of your drain. Why? I don't know! But I might be able to help with this:
the sink stopper is difficult to remove so not sure how to check. It seems to be hooked in very tightly.

If the stopper won't pull up then the installer tied it down. To free the stopper, push down on the knob raising the stopper to the drain position. Now go underneith the lavatory and locate a knured nut with a rod sticking out of it. Take a pair of pliers and free the nut just enough for it to turn by hand. STOP! Now hand loosen the nut enough so the nut and rod just begains to clear. STOP!
Now GENTLY, while you're pulling up on the stopper pull the nut and rod out SLOWLY and STOP when the stopper clears enough to remove. CAREFUL! Don't let it pull all the way out. Now tighten the nut by hand and snug it up with a pair of pliers. You may now just drop the stopper in and it will continue to ride on top of the rod so if you ever have to remove it you can.
Hope this helps . Tom