I had a similar problem; it turned out that it was a leak from the washer fluid reservoir. The reservoir has a spot for a second washer fluid pump (which my car does not have) so the opening for the 2nd pump was blocked with a filler plug. The plug had disintegrated and fluid would randomly leak out and form a mystery puddle under the car. For some reason, it would hold fluid for days and then suddenly empty itself in a matter of minutes when I wasn't looking, so it took a while to find the problem.

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD