RG wrote:

The OP claims to have eliminated all on-board systems as the source due to
draining all tanks. Maybe so, but onboard water systems would be highly
suspect, since the leak has appeared immediately after post-winterization
recommissioning.

If the onboard systems, like potable water tanks etc are all filled
using public water, you can test for chlorine using a very cheap test
kit from a pool supply store.
..