Hi John, welcome to the forum! Leak detection is an inexact science, what's already been suggested (plug lines and bucket test) is the usual place to start.
I have a suggestion or two for you to try:
Check the inside walls and especially the bottom of the skimmer (s) for hairline cracks - easy enough to do as you'll be plugging them off.
Check the backwash port on your multiport (if you don't have one this will be very hard to do ) for seepage - while you'rre there check for wet spots on the filter pad.

You're not loosing a ton of water/ day so the leak is going to be small, unfortunately that makes it harder to find, if it's not something obvious - however any water loss is too much.
Since I don't know where you are I have to ask about your rainfall this season, lots of flooding like in the NE? If so, all the rain may have prevented you from noticing a leak until last week - also lots of rain might have made the hydrostat pop and perhaps it didn't properly reseal (good thing you've got the SCUBA set).
Please let us know how the tests go - we're falling over each other trying to help - Good Luck -- Waste