Mystery Puddle After First Thaw

The previous owner winterized our 2021 400 with antifreeze following the Nautilus instructions. He drained but did not blow out the lines beforehand.

After the first warm day, I found a puddle of clean water (maybe two cups) under the trailer on the Coroplast forward of the fresh water tank and aft of the gray tank.  I could find no wet spots along the undercarriage or tanks.

I opened the fresh water tank drain and perhaps a quart of water flowed out.  I opened the gray tank and perhaps a quart or more of gray water came out.  I poured antifreeze down each drain one at at time and found no leaks.  I emptied the antifreeze from the gray water tank after checking each drain.  All seemed Ok.

A week later, we had another warm day and I once again found a small puddle (one cup?) at the same spot with no wet signs on the undercarriage or tanks.  

I guess I will have to wait another two months until I can dewinterize and fill the lines.  My fear is that a water line somehow retained some water, froze, and broke as I cannot think of any other source.  Anything else to consider?