Trust your daughter's assessment that the liquid is water. The touch and smell test is usually quite conclusive. I like to go straight for the taste test myself.

As for where it's coming from, you might research what drains exist for the power hardtop? Unfortunately I've never laid eyes the mechanism but there must be a low point that is open to the elements. Perhaps there's some water trapped by a clogged drain and it's slowly or periodically freeing itself from the coddled confines of the C70. Check the carpets carefully for water also.

If you're (un)lucky there's a leak in the cabin draining onto the fuel pump controller (PEM) under the left rear seat and that's what's causing the hard starting. <--- A true armchair diagnosis right there, YMMV, refer to my earlier taste test statement...

Here's to hoping for an easy diagnosis and even easier resolution to your mystery!

-Will